Astrocytes modulate distribution and neuronal signaling of leptin in the hypothalamus of obese A vy mice

Insulin and the blood-brain barrier

The blood-brain barrier (BBB) predominantly regulates insulin transport into and levels within the brain. The BBB is also an important site of insulin binding and mediator of insulin receptor (INSR) signaling. The insulin transporter is separate from the INSR, highlighting the important, unique role of each protein in this structure. After a brief introduction on the structure of insulin and the INSR, we discuss the importance of insulin interactions at the BBB, the properties of the insulin transporter and the role of the BBB insulin transporter in various physiological conditions. We go on to further describe insulin BBB signaling and the impact not only within brain endothelial cells but also the cascade into other cell types within the brain. We close with future considerations to advance our knowledge about the importance of insulin at the BBB.

Keratinocyte-Derived Cytokine in the Hippocampus Disrupts Extinction of Conditioned Fear Memory in Tumor-Bearing Mice

While patients with cancer show a higher prevalence of psychiatric disorders than the general population, the mechanism underlying this interaction remains unclear. The present study examined whether tumor-bearing (TB) mice show psychological changes using the conditioned fear paradigm and the role of cytokines in these changes. TB mice were established by transplantation with mouse osteosarcoma AXT cells. These TB mice were then found to exhibit disruption in extinction of conditioned fear memory. Eighteen cytokines in serum were increased in TB mice, among which i.c.v. injection of interleukin (IL)-1β and IL-6 strengthened fear memory in normal mice. Contents of IL-17 and keratinocyte-derived cytokine (KC) in the amygdala and KC in the hippocampus were increased in TB mice. KC mRNA in both the amygdala and hippocampus was also increased in TB mice, and i.c.v. injection of KC dose-dependently strengthened fear memory in normal mice. In addition, injection of IL-1β, but not IL-6, increased KC mRNA in the amygdala and hippocampus. In TB mice KC mRNA was increased in both astrocytes and microglia of the amygdala and hippocampus. The microglia inhibitor minocycline, but not the astrocyte inhibitor fluorocitrate, alleviated disruption in extinction of conditioned fear memory in TB mice. Microinjection of KC into the hippocampus, but not into the amygdala, increased fear memory in normal mice. These findings indicate that TB mice show an increase in serum cytokines, including IL-1β, that increases KC production in microglia of the hippocampus, which then disrupts extinction of fear memory.

Deconstruction of a hypothalamic astrocyte-white adipocyte sympathetic axis that regulates lipolysis in mice

The role of non-neuronal glial cells in the regulation of adipose sympathetic nerve activity and adipocyte functions such as white adipose tissue lipid lipolysis is poorly understood. Here, we combine chemo/optogenetic manipulations of medio-basal hypothalamic astrocytes, real-time fiber photometry monitoring of white adipose tissue norepinephrine (NE) contents and nerve activities, electrophysiological recordings of local sympathetic inputs to inguinal white adipose tissue (iWAT), and adipose tissue lipid lipolytic assays to define the functional roles of hypothalamic astrocytes in the regulation of iWAT sympathetic outflow and lipolysis. Our results show that astrocyte stimulation elevates iWAT NE contents, excites sympathetic neural inputs and promotes lipolysis. Mechanistically, we find that sympathetic paravertebral ganglia (PG) partake in those astrocyte effects. We also find that astrocyte stimulation excites pro-opiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus (ARH), and chemogenetic inhibition of POMC neurons blunts the effects induced by astrocyte stimulation. While we cannot exclude potential roles played by other cell populations such as microglia, our findings in this study reveal a central astrocyte-peripheral adipocyte axis modulating sympathetic drive to adipose tissues and adipocyte functions, one that might serve as a target for therapeutic intervention in the treatment of obesity.

Leptin and adiponectin concentrations in infants with low birth weight: relationship with maternal health and postnatal growth

Health in pregnancy and infancy can affect the risk of chronic non-communicable diseases. We aimed to describe leptin and adiponectin concentrations in low birth weight (LBW) infants and identify possible associations with maternal nutritional status, adequacy for gestational age, nutritional recovery, and current dietary intake. A cross-sectional study with LBW infants (9-12 months) including maternal background and pre-pregnancy nutritional condition was performed. From the Infants: anthropometry at birth and current was expressed as z-score (weight: WAZ, length, head circumference), nutritional recovery, dietary intake, leptin, and adiponectin blood concentrations. The mean age of the 54 infants was 10.0 ± 1.5 months, 32 (59.3%) were female, 36 (66.7%) preterm, 23 (42.6%) small for gestational age (SGA), and 25 pregnancies (46.3%) were twin. Almost all (98%) of the infants intake energy and protein above the recommendation, and 47 (87.6%) consumed ultra-processed foods. At the time of the assessment, 8 (14.8%) were overweight and 4 (7.4%) had short stature. SGA infants showed faster weight recovery (WAZ 1.54; 95% CI 1.17, 1.91; p = 0.001), higher leptin's concentration (3.0 ng/ml (1.7, 3.0) versus 1.6 ng/ml (0.9, 2.6); p = 0.032)), and leptin/adiponectin ratio (0.13 ± 0.08 versus 0.07 ± 0.07; p = 0.018). The pre-gestational BMI was a modifier of the effect of WAZ on leptin levels (p = 0.027) in LBW infants. Higher pre-gestational BMI increased the effect of WAZ variation (birth and current) on leptin levels. Concluding, LBW infants showed early changes in leptin and adiponectin concentrations, influenced by maternal (pre-gestational BMI), intrauterine (gestational age adequacy - SGA), and postnatal weight gain. This combination of factors may increase the risk of NCD for this group of children.

Hypothalamic Astrocytes as a Specialized and Responsive Cell Population in Obesity

Astrocytes are a type of glial cell anatomically and functionally integrated into the neuronal regulatory circuits for the neuroendocrine control of metabolism. Being functional integral compounds of synapses, astrocytes are actively involved in the physiological regulatory aspects of metabolic control, but also in the pathological processes that link neuronal dysfunction and obesity. Between brain areas, the hypothalamus harbors specialized functional circuits that seem selectively vulnerable to metabolic damage, undergoing early cellular rearrangements which are thought to be at the core of the pathogenesis of diet-induced obesity. Such changes in the hypothalamic brain region consist of a rise in proinflammatory cytokines, the presence of a reactive phenotype in astrocytes and microglia, alterations in the cytoarchitecture and synaptology of hypothalamic circuits, and angiogenesis, a phenomenon that cannot be found elsewhere in the brain. Increasing evidence points to the direct involvement of hypothalamic astrocytes in such early metabolic disturbances, thus moving the study of these glial cells to the forefront of obesity research. Here we provide a comprehensive review of the most relevant findings of molecular and pathophysiological mechanisms by which hypothalamic astrocytes might be involved in the pathogenesis of obesity.

Olanzapine-Induced Activation of Hypothalamic Astrocytes and Toll-Like Receptor-4 Signaling via Endoplasmic Reticulum Stress Were Related to Olanzapine-Induced Weight Gain

The antipsychotic drug olanzapine is associated with serious obesity side effects. Hypothalamic astrocytes and associated toll-like receptor-4 (TLR4) signaling play an essential role in obesity pathogenesis. This study investigated the effect of olanzapine on astrocytes and TLR4 signaling both in vitro and in the rat hypothalamus and their potential role in olanzapine-induced weight gain. We found that olanzapine treatment for 24 h dose-dependently increased cell viability, increased the protein expression of astrocyte markers including glial fibrillary acidic protein (GFAP) and S100 calcium binding protein B (S100B), and activated TLR4 signaling in vitro. In rats, 8- and 36-day olanzapine treatment caused weight gain accompanied by increased GFAP and S100B protein expression and activated TLR4 signaling in the hypothalamus. These effects still existed in pair-fed rats, suggesting that these effects were not secondary effects of olanzapine-induced hyperphagia. Moreover, treatment with an endoplasmic reticulum (ER) stress inhibitor, 4-phenylbutyrate, inhibited olanzapine-induced weight gain and ameliorated olanzapine-induced changes in hypothalamic GFAP, S100B, and TLR4 signaling. The expression of GFAP, S100B, and TLR4 correlated with food intake and weight gain. These findings suggested that olanzapine-induced increase in hypothalamic astrocytes and activation of TLR4 signaling were related to ER stress, and these effects may be related to olanzapine-induced obesity.

Altered Brain Leptin and Leptin Receptor Expression in the 5XFAD Mouse Model of Alzheimer's Disease

Alzheimer's disease (AD) is a complex neurodegenerative disorder characterized by the accumulation of amyloid plaques and neurofibrillary tangles. Interestingly, individuals with metabolic syndromes share some pathologies with those diagnosed with AD including neuroinflammation, insulin resistance and cognitive deficits. Leptin, an adipocyte-derived hormone, regulates metabolism, energy expenditure and satiety via its receptor, LepR. To investigate the possible involvement of leptin in AD, we examined the distribution of leptin and LepR in the brains of the 5XFAD mouse model of AD, utilizing immunofluorescent staining in young (10-12-weeks; n = 6) and old (48-52-weeks; n = 6) transgenic (Tg) mice, together with age-matched wild-type (WT) controls for both age groups (young-WT, n = 6; old-WT, n = 6). We also used double immunofluorescent staining to examine the distribution of leptin and leptin receptor expression in astrocytes. In young 5XFAD, young-WT and old-WT mice, we observed neuronal and endothelial expression of leptin and LepR throughout the brain. However, neuronal leptin and LepR expression in the old 5XFAD brain was significantly diminished. Reduced neuronal leptin and LepR expression was accompanied by plaque loading and neuroinflammation in the AD brain. A marked increase in astrocytic leptin and LepR was also observed in old 5XFAD mice compared to younger 5XFAD mice. We postulate that astrocytes may utilize LepR signalling to mediate and drive their metabolically active state when degrading amyloid in the AD brain. Overall, these findings provide evidence of impaired leptin and LepR signalling in the AD brain, supporting clinical and epidemiological studies performed in AD patients.

Leptin-Mediated Sympathoexcitation in Obese Rats: Role for Neuron-Astrocyte Crosstalk in the Arcuate Nucleus

Introduction

Accumulated evidence indicates that obesity is associated with enhanced sympathetic activation. Hypothalamic leptin-mediated signaling may contribute to the exaggerated sympathoexcitation of obesity. The goal of this study was to investigate the "neuron-astrocyte" interaction affecting leptin-mediated sympathoexcitation within the arcuate nucleus (ARCN) of the hypothalamus in obese rats.

Methods and Results

Obesity was induced by high-fat diet (HFD, 42% of calories from fat) in Sprague Dawley rats. Twelve weeks of HFD produced hyperleptinemia, hyperlipidemia, and insulin resistance. In anesthetized rats, microinjections of leptin into the ARCN induced increases in heart rate (HR), renal sympathetic nerve activity (RSNA), and mean arterial pressure (MAP) in both control and HFD rats. However, microinjections of leptin in HFD rats elicited higher responses of RSNA and arterial pressure than control-fed rats. It also caused the inhibition of astrocytes within the ARCN using an astrocytic metabolic inhibitor, fluorocitrate, and reduced leptin-induced sympathetic activity and blood pressure responses. Moreover, the expression of the leptin receptor in the ARCN of HFD-fed rats was significantly increased compared to rats fed a control diet. Immunohistochemistry analysis revealed leptin receptor localization from both neurons and astrocytes of the ARCN. HFD rats exhibited increased protein expression of glial fibrillary acidic protein (GFAP) in the ARCN. We also found that the expression of astrocyte-specific glutamate transporters and excitatory amino acid transporter 1 (EAAT1) and 2 (EAAT2) were decreased within the ARCN of the HFD rats. In cultured astrocytic C6 cells, 24 h of leptin treatment increased the protein expression of GFAP and reduced the expression of EAAT1 and EAAT2.

Conclusion

The results suggest that central leptin signaling occurs via neuron-astrocyte interactions in the ARCN and contributing to the exaggerated sympathoexcitation observed in obese rats. The effects may be mediated by the action of leptin on regulating astrocytic glutamate transporters within the ARCN of the hypothalamus.

Memory impairments and increased GFAP expression in hippocampal astrocytes following hypercaloric diet in rats

ABSTRACT Objective: Hypothalamic inflammation and glial fibrillary acidic protein (GFAP) overexpression in astrocytes are well described in obese animals, as are some cognitive and memory deficits. As the hippocampus plays important roles in the consolidation of information, this investigation aimed to observe the memory function and the astrocyte expression of GFAP in the hippocampus of rats that received either a hypercaloric or a normocaloric diet. Methods: Adult male Wistar rats received a high-fat (cafeteria) or a standard diet for 60 days. On the 61st day, the rats were submitted to the novel object recognition (NOR) test at three and 24 hours after the first contact with objects, to assess short-term and long-term memory, respectively. Thereafter, the rats were euthanized and their brains were collected for GFAP immunohistochemical investigation in the hippocampus (CA1, CA2, CA3 areas) and hypothalamus (periventricular and arcuate nuclei). Astrocytic reactivity was assessed by morphometry. Different white adipose tissue depots and brown adipose tissue were weighed to calculate the adiposity index. Results: The hypercaloric diet increased body weight gain, adiposity index, white adipose tissue weight (epididymal, subcutaneous and retroperitoneal) and brown adipose tissue weight. Rats fed with the hypercaloric diet showed short-term and long-term memory impairments in the NOR test, as well as increased GFAP expression in astrocytes from all analyzed hypothalamic and hippocampal areas. Conclusion: This astrogliosis suggests that the neuroinflammatory response also occurs in the hippocampus and may be involved in the memory losses observed in obese/overweight animals.

The Effects of Leptin on Glial Cells in Neurological Diseases

It is known that various endocrine modulators, including leptin and ghrelin, have neuroprotective roles in neurological diseases. Leptin is a hormone produced by adipocytes and was originally identified as a gene related to obesity in mice. The leptin receptors in the hypothalamus are the main target for the homeostatic regulation of body weight. Recent studies have demonstrated that leptin receptors are also expressed in other regions of the central nervous system (CNS), such as the hippocampus, cerebral cortex, and spinal cord. Accordingly, these studies identified the involvement of leptin in the regulation of neuronal survival and neural development. Furthermore, leptin has been shown to have neuroprotective functions in animal models of neurological diseases and demyelination. These observations also suggest that dysregulation of leptin signaling may be involved in the association between neurodegeneration and obesity. In this review, we summarize novel functions of leptin in animal models of neurodegenerative diseases. Specifically, we focus on the emerging evidence for the role of leptin in non-neuronal cells in the CNS, including astrocytes, microglia, and oligodendrocytes. Understanding leptin-mediated neuroprotective signals and molecular mechanisms underlying remyelination will be helpful to establish therapeutic strategies against neurological diseases.

Inhibition of activated astrocyte ameliorates lipopolysaccharide- induced depressive-like behaviors

BACKGROUND

Numerous studies indicate that inflammation plays important roles in the development of depression. Astrocytes are crucial regulators of immune response in the central nervous system, and strongly activated by pro-inflammatory cytokines. We hypothesized that inhibition of activated astrocytes contributed to ameliorate depressive-like symptoms.

METHODS

This study evaluated the antidepressant-like effect of inhibition of activated astrocytes, by a well-established astrocyte inactivator fluorocitrate (FC), on a lipopolysaccharide (LPS)-induced model of depression. Forced swim test (FST), tail suspension test (TST) and sucrose preference test were used to assess depressive-like behaviors. The expression of fibrillary acidic protein (GFAP), brain-derived neurotrophic factor (BDNF) and neuroinflammation were determined in the hippocampus and cortex.

RESULTS

The results demonstrated that LPS increased immobility time in the TST and FST, reduced sucrose preference as well. LPS also enhanced the expression of IL-1β, TNF-α, iNOS and GFAP, accompanying with decreased expression of BDNF in the hippocampus and cortex. Inhibition of activated astrocytes by FC significantly prevented LPS- induced alteration in the FST, TST and sucrose preference test. Moreover, in the hippocampus and cortex, inhibition of activated astrocytes by FC significantly attenuated increases of neuroinflammation and GFAP whereas reversed decrease of BDNF in LPS- challenged depression.

CONCLUSIONS

Taken together, the results suggest that inhibition of activated astrocytes ameliorates LPS-induced depressive-like behavior, providing the first evidence that inhibition of activated astrocytes might represent a novel therapeutic target for depression.

Leptin: role over central nervous system in epilepsy

Adipose tissue is a dynamic organ with different effects on the body. Many of these effects are mediated by leptin, a hormone strongly involved in regulation of feeding and energy metabolism. It has an important role as a mediator of neuronal excitatory activity and higher brain functions. The aim of this study was to review the association between leptin and cerebral neuronal function, in particular its anticonvulsant or convulsant effects and the possible therapeutic role for treating epilepsy. For this purpose, the databases Pubmed, Science Direct, Elsevier, ResearchGate and Scielo were searched to identify experimental studies, reviews and systematic review articles, published in English, Spanish or Portuguese. Experimental studies and the presence of leptin receptors in nervous system sites other than the hypothalamus suggest an influence on higher brain functions. Indeed several animal studies have demonstrated a role of these channels in epileptiform activity as both anticonvulsive and convulsive effects have been found. The reason for these discrepancies is unclear but provides clear evidence of a potential role of leptin and leptin therapy in epileptiform activity. The association between leptin and brain function demonstrates the importance of peripheral metabolic hormones on central nervous system and opens a new way for the development of novel therapeutic interventions in diseases like epilepsy. Nevertheless further investigations are important to clarify the dynamics and diverse actions of leptin on excitatory regulation in the brain.

Reduced astrocytic expression of GFAP in the offspring of female rats that received hypercaloric diet

Introduction: Obesity promotes hypothalamic inflammation and local morphological changes in astrocytes, including the increased expression of the astrocytic biomarker glial fibrillary acidic protein (GFAP), which is seen as a sign of neuroinflammation.Objective: This study aimed to observe the astrocytic expression of GFAP in different brain areas from female rats that received a hypercaloric (HD) or a normocaloric (ND) diet during puberty (F0 generation) as well as in their male pups (F1 generation).Methods: Female rats received highly palatable HD (Ensure®) or ND from postnatal day (PND) 23-65. On PND90-95, some were euthanized for the immunohistochemical study and some were mated to obtain the F1 generation. Male pups were immunochallenged on PND50 with lipopolysaccharide (LPS, 100 μg/kg) or 0.9% saline solution (1 mL/kg) intraperitoneal injection. Body weight (BW) and retroperitoneal fat weight (RFW) were recorded on PND95 for F0 generation and on PND50 for F1 generation. GFAP expression for both generations was assessed by morphometry in the parietal/frontal cortex, corpus callosum, nucleus accumbens, arcuate/periventricular nuclei of hypothalamus, pons, molecular/granular layers of cerebellum.Results: Female rats fed with HD presented a significant increase in the GFAP expression in all evaluated areas as well as in the RFW. Male rats born from mothers that received HD showed decreased GFAP expression, BW and RFW when treated with LPS in relation to those from mothers fed with ND.Discussion: HD induced astrogliosis in several brain areas in females from F0 generation and an adaptive phenotypic change of decreased GFAP expression in males from F1 generation after LPS challenge.

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.

Glial Fatty Acid-Binding Protein 7 (FABP7) Regulates Neuronal Leptin Sensitivity in the Hypothalamic Arcuate Nucleus

The hypothalamus is involved in the regulation of food intake and energy homeostasis. The arcuate nucleus (ARC) and median eminence (ME) are the primary hypothalamic sites that sense leptin and nutrients in the blood, thereby mediating food intake. Recently, studies demonstrating a role for non-neuronal cell types, including astrocytes and tanycytes, in these regulatory processes have begun to emerge. However, the molecular mechanisms involved in these activities remain largely unknown. In this study, we examined in detail the localization of fatty acid-binding protein 7 (FABP7) in the hypothalamic ARC and sought to determine its role in the hypothalamus. We performed a phenotypic analysis of diet-induced FABP7 knockout (KO) obese mice and of FABP7 KO mice treated with a single leptin injection. Immunohistochemistry revealed that FABP7⁺ cells are NG2⁺ or GFAP⁺ in the ARC and ME. In mice fed a high-fat diet, weight gain and food intake were lower in FABP7 KO mice than in wild-type (WT) mice. FABP7 KO mice also had lower food intake and weight gain after a single injection of leptin, and we consistently confirmed that the number of pSTAT3⁺ cells in the ARC indicated that the leptin-induced activation of neurons was significantly more frequent in FABP7 KO mice than in WT mice. In FABP7 KO mice-derived primary astrocyte cultures, the level of ERK phosphorylation was lower after leptin treatment. Collectively, these results indicate that in hypothalamic astrocytes, FABP7 might be involved in sensing neuronal leptin via glia-mediated mechanisms and plays a pivotal role in controlling systemic energy homeostasis.

Propentofylline decreases hypothalamic astrogliosis induced by hypercaloric diet in the rat

ABSTRACT Obesity is associated with a chronic and low-grade inflammatory response in the hypothalamus, where astrogliosis occurs with the upregulation of the astrocyte structural protein GFAP. As propentofylline (PPF) has inhibitory effects on astrocyte and microglial activation during inflammation, this study aimed to investigate if this xanthine derivative could decrease the astrocyte reaction induced by a hypercaloric diet (HD). Male Wistar rats were divided into four groups: NDS – rats receiving a normocaloric diet (ND) and daily saline solution; NDP – rats receiving ND and daily PPF (12.5 mg/kg/day, intraperitoneal route); HDS – rats receiving HD and saline solution, HDP – rats receiving HD and PPF. On the 21st day, rats were anesthetized, and perfused, and brains were collected for GFAP immunohistochemical study in the hypothalamus. Results showed that HD induced increased weight gain and hypothalamic astrogliosis. Propentofylline decreased the expression of GFAP in the HDP group, although it did not affect the weight gain induced by this diet.

Dietary fats promote functional and structural changes in the median eminence blood/spinal fluid interface-the protective role for BDNF

BACKGROUND

The consumption of large amounts of dietary fats activates an inflammatory response in the hypothalamus, damaging key neurons involved in the regulation of caloric intake and energy expenditure. It is currently unknown why the mediobasal hypothalamus is the main target of diet-induced brain inflammation. We hypothesized that dietary fats can damage the median eminence blood/spinal fluid interface.

METHODS

Swiss mice were fed on a high-fat diet, and molecular and structural studies were performed employing real-time PCR, immunoblot, immunofluorescence, transmission electron microscopy, and metabolic measurements.

RESULTS

The consumption of a high fat diet was sufficient to increase the expression of inflammatory cytokines and brain-derived neurotrophic factor in the median eminence, preceding changes in other circumventricular regions. In addition, it led to an early loss of the structural organization of the median eminence β1-tanycytes. This was accompanied by an increase in the hypothalamic expression of brain-derived neurotrophic factor. The immunoneutralization of brain-derived neurotrophic factor worsened diet-induced functional damage of the median eminence blood/spinal fluid interface, increased diet-induced hypothalamic inflammation, and increased body mass gain.

CONCLUSIONS

The median eminence/spinal fluid interface is affected at the functional and structural levels early after introduction of a high-fat diet. Brain-derived neurotrophic factor provides an early protection against damage, which is lost upon a persisting consumption of large amounts of dietary fats.

Non-Neuronal Cells in the Hypothalamic Adaptation to Metabolic Signals

Although the brain is composed of numerous cell types, neurons have received the vast majority of attention in the attempt to understand how this organ functions. Neurons are indeed fundamental but, in order for them to function correctly, they rely on the surrounding "non-neuronal" cells. These different cell types, which include glia, epithelial cells, pericytes, and endothelia, supply essential substances to neurons, in addition to protecting them from dangerous substances and situations. Moreover, it is now clear that non-neuronal cells can also actively participate in determining neuronal signaling outcomes. Due to the increasing problem of obesity in industrialized countries, investigation of the central control of energy balance has greatly increased in attempts to identify new therapeutic targets. This has led to interesting advances in our understanding of how appetite and systemic metabolism are modulated by non-neuronal cells. For example, not only are nutrients and hormones transported into the brain by non-neuronal cells, but these cells can also metabolize these metabolic factors, thus modifying the signals reaching the neurons. The hypothalamus is the main integrating center of incoming metabolic and hormonal signals and interprets this information in order to control appetite and systemic metabolism. Hence, the factors transported and released from surrounding non-neuronal cells will undoubtedly influence metabolic homeostasis. This review focuses on what is known to date regarding the involvement of different cell types in the transport and metabolism of nutrients and hormones in the hypothalamus. The possible involvement of non-neuronal cells, in particular glial cells, in physiopathological outcomes of poor dietary habits and excess weight gain are also discussed.

Differential hypothalamic leptin sensitivity in obese rat offspring exposed to maternal and postnatal intake of chocolate and soft drink

Background/objective:

Intake of high-energy foods and maternal nutrient overload increases the risk of metabolic diseases in the progeny such as obesity and diabetes. We hypothesized that maternal and postnatal intake of chocolate and soft drink will affect leptin sensitivity and hypothalamic astrocyte morphology in adult rat offspring.

Methods:

Pregnant Sprague-Dawley rats were fed ad libitum chow diet only (C) or with chocolate and high sucrose soft drink supplement (S). At birth, litter size was adjusted into 10 male offspring per mother. After weaning, offspring from both dietary groups were assigned to either S or C diet, giving four groups until the end of the experiment at 26 weeks of age.

Results:

As expected, adult offspring fed the S diet post weaning became obese (body weight: P<0.01, %body fat per kg: P<0.001) and this was due to the reduced energy expenditure (P<0.05) and hypothalamic astrogliosis (P<0.001) irrespective of maternal diet. Interesting, offspring born to S-diet-fed mothers and fed the S diet throughout postnatal life became obese despite lower energy intake than controls (P<0.05). These SS offspring showed increased feed efficiency (P<0.001) and reduced fasting pSTAT3 activity (P<0.05) in arcuate nucleus (ARC) compared with other groups. The findings indicated that the combination of the maternal and postnatal S-diet exposure induced persistent changes in leptin signalling, hence affecting energy balance. Thus, appetite regulation was more sensitive to the effect of leptin than energy expenditure, suggesting differential programming of leptin sensitivity in ARC in SS offspring. Effects of the maternal S diet were normalized when offspring were fed a chow diet after weaning.

Conclusions:

Maternal intake of chocolate and soft drink had long-term consequences for the metabolic phenotype in the offspring if they continued on the S diet in postnatal life. These offspring displayed obesity despite lowered energy intake associated with alterations in hypothalamic leptin signalling.

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.

Neonatal Nicotine Exposure Leads to Hypothalamic Gliosis in Adult Overweight Rats

Astrocytes and microglia, the immune competent cells of central nercous system, can be activated in response to metabolic signals such as obesity and hyperleptinaemia. In rats, maternal exposure to nicotine during lactation leads to central obesity, hyperleptinaemia, leptin resistance and alterations in hypothalamic neuropeptides in the offspring during adulthood. In the present study, we studied the activation of astrocytes and microglia, as well as the pattern of inflammatory mediators, in adult offspring of this experimental model. On postnatal day 2 (P2), osmotic minipumps releasing nicotine (NIC) (-6 mg/kg/day) or saline for 14 days were s.c. implanted in dams. Male offspring were killed on P180 and hypothalamic immunohistochemistry, retroperitoneal white adipose tissue (WAT) polymerase chain reaction analysis and multiplex analysis for plasma inflammatory mediators were carried out. At P180, NIC astrocyte cell number was higher in the arcuate nucleus (ARC) (medial: +82%; lateral: +110%), in the paraventricular nucleus (PVN) (+144%) and in the lateral hypothalamus (+121%). NIC glial fibrillary acidic protein fibre density was higher in the lateral ARC (+178%) and in the PVN (+183%). Interleukin-6 was not affected in the hypothalamus. NIC monocyte chemotactic protein 1 was only higher in the periventricular nucleus (+287%). NIC microglia (iba-1-positive) cell number was higher (+68%) only in the PVN, as was the chemokine (C-X3-C motif) receptor 1 density (+93%). NIC interleukin-10 was lower in the WAT (-58%) and plasma (-50%). Thus, offspring of mothers exposed to nicotine during lactation present hypothalamic astrogliosis at adulthood and microgliosis in the PVN.

From blood to brain through BBB and astrocytic signaling

In this Festschrift, I discuss the career and guiding principles to which Abba J. Kastin has adhered during the last 20 years we worked together. I briefly describe the history of our joint laboratory group, the context of studies of peptide permeation across the blood-brain barrier (BBB), and newer developments in the BBB Group as Abba steps down after serving 35 years as the founding Editor-in-Chief for Peptides. Abba's BBB studies on peptides have contributed to concepts in the neuroendocrinology of feeding and developed information on molecular trafficking across BBB cells. The astroglial leptin signaling studies and the interactions of sleep and BBB are two major directions, whereas the long-term MIF-1 project demarcates a tortuous road on translational research.

Peptides and the blood-brain barrier

The demonstration that peptides and regulatory proteins can cross the blood-brain barrier (BBB) is one of the major contributions of Dr. Abba J. Kastin. He was the first to propose that peptides could cross the BBB, the first to show that an endogenous peptide did so, and the first to describe a saturable transport system at the BBB for peptides. His work shows that in crossing the BBB, peptides and regulatory proteins act as informational molecules, informing the brain of peripheral events. Brain-to-blood passage helps to control levels of peptides with the brain and can deliver information in the brain-to-blood direction. He showed that the transporters for peptides and proteins are not static, but respond to developmental and physiological changes and are affected by disease states. As such, the BBB is adaptive to the needs of the CNS, but when that adaption goes awry, the BBB can be a cause of disease. The mechanisms by which peptides and proteins cross the BBB offer opportunities for drug delivery of these substances or their analogs to the brain in the treatment of diseases of the central nervous system.

Astrocytes control food intake by inhibiting AGRP neuron activity via adenosine A1 receptors

It is well recognized that feeding behavior in mammals is orchestrated by neurons within the medial basal hypothalamus. However, it remains unclear whether food intake is also under the control of glial cells. Here, we combine chemical genetics, cell-type-specific electrophysiology, pharmacology, and feeding assays to show that stimulation of astrocytes within the medial basal hypothalamus reduces both basal- and ghrelin-evoked food intake. This occurs by a mechanism of adenosine-mediated inactivation of the orexigenic agouti-related peptide (AGRP) neurons in the hypothalamic arcuate nucleus (ARC) via adenosine A1 receptors. Our data suggest that glial cells participate in regulating food intake by modulating extracellular levels of adenosine. These findings reveal the existence of a glial relay circuit that controls feeding behavior, one that might serve as a target for therapeutic intervention in the treatment of appetite disorders.

Role of Astrocytes in Leptin Signaling

To test the hypothesis that astrocytic leptin signaling induces an overall potentiation of the neuronal response to leptin, we generated a new line of astrocyte-specific leptin receptor knockout (ALKO-Δ1) mice in which no leptin receptor is expressed in astrocytes. Corresponding to cell-specific Cre recombinase expression in hypothalamic astrocytes but not neurons, this new strain of ALKO mice had attenuated pSTAT3 signaling in the arcuate nucleus of the hypothalamus 30 min after intracerebroventricular delivery of leptin. In response to high-fat diet for 2 months, the ALKO mice showed a greater increase of percent fat and blood leptin concentration. This coincided with a mild reactive gliosis in the hypothalamus. Overall, the absence of leptin receptors in astrocytes attenuated hypothalamic pSTAT3 signaling, induced a mild reactive morphology, and promoted the development of diet-induced obesity. We conclude that leptin signaling in astrocytes is essential for the homeostasis of neuroendocrine regulation in obesity.

Early weaning by maternal prolactin inhibition leads to higher neuropeptide Y and astrogliosis in the hypothalamus of the adult rat offspring

The suppression of prolactin production with bromocriptine (BRO) in the last 3 d of lactation reduces milk yield (early weaning) and increases the transfer of leptin through the milk, causing hyperleptinaemia in pups. In adulthood, several changes occur in the offspring as a result of metabolic programming, including overweight, higher visceral fat mass, hypothyroidism, hyperglycaemia, insulin resistance, hyperleptinaemia and central leptin resistance. In the present study, we investigated whether overweight rats programmed by early weaning with maternal BRO treatment have hypothalamic alterations in adulthood. We analysed the expression of neuropeptide Y (NPY), cocaine- and amphetamine-regulated transcript (CART), pro-opiomelanocortin (POMC) and α-melanocyte-stimulating hormone (α-MSH) by immunohistochemistry in the following hypothalamic nuclei: medial and lateral arcuate nucleus (ARC); paraventricular nucleus (PVN); lateral hypothalamus (LH). Additionally, we sought to determine whether these programmed rats exhibited hypothalamic inflammation as indicated by astrogliosis. NPY immunostaining showed a denser NPY-positive fibre network in the ARC and PVN (+82 % in both nuclei) of BRO offspring. Regarding the anorexigenic neuropeptides, no difference was found for CART, POMC and α-MSH. The number of astrocytes was higher in all the nuclei of BRO rats. The fibre density of glial fibrillary acidic protein was also increased in both medial and lateral ARC (6·06-fold increase and 9·13-fold increase, respectively), PVN (5·75-fold increase) and LH (2·68-fold increase) of BRO rats. We suggest that early weaning has a long-term effect on the expression of NPY as a consequence of developmental plasticity, and the presence of astrogliosis indicates hypothalamic inflammation that is closely related to overweight and hyperleptinaemia observed in our model.

Role of non-neuronal cells in body weight and appetite control

The brain is composed of neurons and non-neuronal cells, with the latter encompassing glial, ependymal and endothelial cells, as well as pericytes and progenitor cells. Studies aimed at understanding how the brain operates have traditionally focused on neurons, but the importance of non-neuronal cells has become increasingly evident. Once relegated to supporting roles, it is now indubitable that these diverse cell types are fundamental for brain development and function, including that of metabolic circuits, and they may play a significant role in obesity onset and complications. They participate in processes of neurogenesis, synaptogenesis, and synaptic plasticity of metabolic circuits both during development and in adulthood. Some glial cells, such as tanycytes and astrocytes, transport circulating nutrients and metabolic factors that are fundamental for neuronal viability and activity into and within the hypothalamus. All of these cell types express receptors for a variety of metabolic factors and hormones, suggesting that they participate in metabolic function. They are the first line of defense against any assault to neurons. Indeed, microglia and astrocytes participate in the hypothalamic inflammatory response to high fat diet (HFD)-induced obesity, with this process contributing to inflammatory-related insulin and leptin resistance. Moreover, HFD-induced obesity and hyperleptinemia modify hypothalamic astroglial morphology, which is associated with changes in the synaptic inputs to neuronal metabolic circuits. Astrocytic contact with the microvasculature is increased by HFD intake and this could modify nutrient/hormonal uptake into the brain. In addition, progenitor cells in the hypothalamus are now known to have the capacity to renew metabolic circuits, and this can be affected by HFD intake and obesity. Here, we discuss our current understanding of how non-neuronal cells participate in physiological and physiopathological metabolic control.

Estrogen, astrocytes and the neuroendocrine control of metabolism

Obesity, and its associated comorbidities such as type 2 diabetes, cardiovascular diseases, and certain cancers, represent major health challenges. Importantly, there is a sexual dimorphism with respect to the prevalence of obesity and its associated metabolic diseases, implicating a role for gonadal hormones. Specifically, estrogens have been demonstrated to regulate metabolism perhaps by acting as a leptin mimetic in the central nervous system (CNS). CNS estrogen receptors (ERs) include ER alpha (ERα) and ER beta (ERβ), which are found in nuclear, cytoplasmic and membrane sites throughout the brain. Additionally, estrogens can bind to and activate a G protein-coupled estrogen receptor (GPER), which is a membrane-associated ER. ERs are expressed on neurons as well as glia, which are known to play a major role in providing nutrient supply for neurons and have recently received increasing attention for their potentially important involvement in the CNS regulation of systemic metabolism and energy balance. This brief overview summarizes data focusing on the potential role of astrocytic estrogen action as a key component of estrogenic modulation responsible for mediating the sexual dimorphism in body weight regulation and obesity.

The effect of partial food deprivation on the astroglia in the dorsal subnucleus of the lateral septum of the rat brain

The effect of 40% partial food deprivation was studied on the immunohistochemically detectable amount of glial fibrillary acidic protein (GFAP) - the specific marker of astroglia - in the dorsal subnucleus of lateral septum (LS) of male, intact and ovariectomized (OVX) female rats. Animals were either fed ad libitum (control) or 40% food deprived for one week, then perfusion-fixed, their brains removed, and serial vibratome sections were processed for the immunocytochemical localization of GFAP. Computeraided densitometry was carried out on digital photographs.The results showed that ovariectomy alone did not exert any effect on the density of GFAPimmunoreactivity (GFAP-IR) as compared to the values detected in intact females. Food deprivation increased the density of GFAP in each experimental group. The difference was most pronounced in males, significant in females and much less in ovariectomized females. Parietal cortex chosen as reference area did not show any increase in the local GFAP-IR.It was previously shown that the dorsal subnucleus of the lateral septum reacts with plastic neurochemical changes to food deprivation. Our results prove that these changes affect not only neuronal but also glial elements.

Loss of astrocytic leptin signaling worsens experimental autoimmune encephalomyelitis

Leptin is commonly thought to play a detrimental role in exacerbating experimental autoimmune encephalomyelitis (EAE) and multiple sclerosis. Paradoxically, we show here that astrocytic leptin signaling has beneficial effects in reducing disease severity. In the astrocyte specific leptin receptor knockout (ALKO) mouse in which leptin signaling is absent in astrocytes, there were higher EAE scores (more locomotor deficits) than in the wildtype counterparts. The difference mainly occurred at a late stage of EAE when wildtype mice showed signs of recovery whereas ALKO mice continued to deteriorate. The more severe symptoms in ALKO mice coincided with more infiltrating cells in the spinal cord and perivascular brain parenchyma, more demyelination, more infiltrating CD4 cells, and a lower percent of neutrophils in the spinal cord 28 days after EAE induction. Cultured astrocytes from wildtype mice showed increased adenosine release in response to interleukin-6 and the hippocampus of wildtype mice had increased adenosine production 28 days after EAE induction, but the ALKO mutation abolished the increase in both conditions. This indicates a role of astrocytic leptin in normal gliotransmitter release and astrocyte functions. The worsening of EAE in the ALKO mice in the late stage suggests that astrocytic leptin signaling helps to clear infiltrating leukocytes and reduce autoimmune destruction of the CNS.

Leptin: a biomarker for sleep disorders?

Leptin, a pleiotropic protein hormone produced mainly by fat cells, regulates metabolic activity and many other physiological functions. The intrinsic circadian rhythm of blood leptin is modulated by gender, development, feeding, fasting, sleep, obesity, and endocrine disorders. Hyperleptinemia is implicated in leptin resistance. To determine the specificity and sensitivity of leptin concentrations in sleep disorders, we summarize here the alterations of leptin in four conditions in animal and human studies: short duration of sleep, sleep fragmentation, obstructive sleep apnea (OSA), and after use of continuous positive airway pressure (CPAP) to treat OSA. The presence and causes of contradictory findings are discussed. Though sustained insufficient sleep lowers fasting blood leptin and therefore probably contributes to increased appetite, obesity and OSA independently result in hyperleptinemia. Successful treatment of OSA by CPAP is predicted to decrease hyperleptinemia, making leptin an ancillary biomarker for treatment efficacy. Current controversies also call for translational studies to determine how sleep disorders regulate leptin homeostasis and how the information can be used to improve sleep treatment.

Uncovering novel roles of nonneuronal cells in body weight homeostasis and obesity

Glial cells, which constitute more than 50% of the mass of the central nervous system and greatly outnumber neurons, are at the vanguard of neuroendocrine research in metabolic control and obesity. Historically relegated to roles of structural support and protection, diverse functions have been gradually attributed to this heterogeneous class of cells with their protagonism in crescendo in all areas of neuroscience during the past decade. However, this dramatic increase in attention bestowed upon glial cells has also emphasized our vast lack of knowledge concerning many aspects of their physiological functions, let alone their participation in numerous pathologies. This minireview focuses on the recent advances in our understanding of how glial cells participate in the physiological regulation of appetite and systemic metabolism as well as their role in the pathophysiological response to poor nutrition and secondary complications associated with obesity. Moreover, we highlight some of the existing lagoons of knowledge in this increasingly important area of investigation.

Hypersomnolence and reduced activity in pan-leptin receptor knockout mice

Excessive obesity correlates with hypersomnolence and impaired cognitive function, presumably induced by metabolic factors and cytokines. Production of the adipokine leptin correlates with the amount of adiposity, and leptin has been shown to promote sleep. To determine whether leptin plays a major role in the hypersomnolence of obesity, we measured sleep architecture in pan-leptin receptor knockout (POKO) mice that do not respond to leptin because of the production of a mutant, non-signaling receptor. The obese POKO mice had more non-rapid eye movement (NREM) sleep and less waking time than their littermate controls. This was mainly seen during the light span, although increased bouts of rapid eye movement sleep were also seen in the dark span. The increase of NREM sleep correlated with the extent of obesity. The POKO mice also had decreased locomotor activity and more immobility in the open field test, but there was no increase of forced immobility nor reduction of sucrose intake as would be seen in depression. The increased NREM sleep and reduced locomotor activity in the POKO mice suggest that it was obesity, rather than leptin signaling, that played a predominant role in altering sleep architecture and activity.

Saturable leptin transport across the BBB persists in EAE mice

We have shown that mice with experimental autoimmune encephalomyelitis (EAE), a model of multiple sclerosis, have upregulated leptin receptor expression in reactive astrocytes of the hippocampus, a region involved in sickness behavior. Leptin can exacerbate EAE when its serum concentration is high. Although leptin receptors in astrocytes modulate leptin transport across cultured endothelial cell monolayers, it is not known how leptin transport in EAE mice is regulated. Here, we determined brain and cervical spinal cord uptake of leptin in early and recovery stages of EAE, after either intravenous delivery or in situ brain perfusion of (125)I-leptin and the vascular marker (131)I-albumin. While increased vascular space and general blood-brain barrier (BBB) permeability after EAE were expected, the specific saturable transport system for leptin crossing the BBB also persisted. Moreover, there was upregulation of leptin transport in hippocampus and cervical spinal cord in the early stage of EAE, shown by higher leptin uptake in these regions and by competitive inhibition with coadministered excess unlabeled leptin. We conclude that EAE induced a time- and region-specific increase of leptin transport. The results provide a link between circulating leptin and enhanced leptin signaling that may play a crucial role in disease progression.

Dietary components in the development of leptin resistance

Classically, leptin resistance has been associated with increased body fat and circulating leptin levels, and the condition is believed to contribute to the onset and/or maintenance of obesity. Although a great deal is known about the central nervous system mechanisms mediating leptin resistance, considerably less is known about the role of diet in establishing and maintaining this altered hormonal state. An exciting new finding has recently been published demonstrating the existence of leptin resistance in normal-weight rats with lean leptin levels by feeding them a high-concentration-fructose diet. This finding has opened the possibility that specific macronutrients may be capable of inducing leptin resistance, independently of the amount of body fat or circulating leptin present in the treated animals. This review describes several lines of research that have recently emerged indicating that specific types of dietary sugars and fats are capable of inducing leptin resistance in experimental rodent models. The results further show that diet-induced leptin resistance is capable of increasing energy intake and elevating body weight gain under appropriate dietary challenges. It appears that biological mechanisms on multiple levels may underlie the dietary induction of leptin resistance, including alterations in the leptin blood-to-brain transport system, in peripheral glucose metabolism, and in central leptin receptor signaling pathways. What is clear from the findings reviewed here is that diet-induced leptin resistance can occur in the absence of elevated circulating leptin levels and body weight, rendering it a potential cause and/or predisposing factor to excess body weight gain and obesity.

Hypothalamic astrocytes in obesity

Obesity is characterized by a chronic and low-grade inflammation in tissues including the hypothalamus. Hypothalamic inflammation is considered an early and determining factor for the onset of obesity, a factor that occurs even before body weight gain. Within the hypothalamus, microglia and astrocytes produce cytokines that drive inflammatory responses. Astrocytes are directly affected by nutrient excess and might play a unique role in promoting hypothalamic inflammatory responses in obesity. This article reviews evidence supporting the role of hypothalamic astrocytes in obesity, and suggests a new approach for neuroendocrine research designed to reveal pathogenesis and develop novel treatment strategies against obesity.

Astrocytic leptin-receptor knockout mice show partial rescue of leptin resistance in diet-induced obesity

To determine how astrocytic leptin signaling regulates the physiological response of mice to diet-induced obesity (DIO), we performed metabolic analyses and hypothalamic leptin signaling assays on astrocytic leptin-receptor knockout (ALKO) mice in which astrocytes lack functional leptin receptor (ObR) signaling. ALKO mice and wild-type (WT) littermate controls were studied at different stages of DIO with measurement of body wt, percent fat, metabolic activity, and biochemical parameters. When fed regular chow, the ALKO mice had similar body wt, percent fat, food intake, heat dissipation, respiratory exchange ratio, and activity as their WT littermates. There was no change in blood concentrations of triglyceride, soluble leptin receptor (sObR), mRNA for leptin and uncoupling protein 1 (UCP1) in adipose tissue, and insulin sensitivity. Unexpectedly, in response to a high-fat diet the ALKO mice had attenuated hyperleptinemia and sObR, a lower level of leptin mRNA in subcutaneous fat, and a paradoxical increase in UCP1 mRNA. Thus, ALKO mice did not show the worsening of obesity that occurs with normal WT mice and the neuronal ObR mutation that results in morbid obesity. The findings are consistent with a competing, counterregulatory model between neuronal and astrocytic leptin signaling.

Protective role of astrocytic leptin signaling against excitotoxicity

Both proconvulsive and anticonvulsive roles of leptin have been reported, suggesting cell-specific actions of leptin in different models of seizure and epilepsy. The goal of our study was to determine the regulation and function of astrocytic leptin receptors in a mouse model of epilepsy and glutamate-induced cytotoxicity. We show that in pilocarpine-challenged mice developing epilepsy with recurrent seizures after a latent period of 2 weeks, hippocampal leptin receptor (ObR) immunofluorescence was increased at 6 weeks. This was more pronounced in astrocytes than in neurons. In cultured astrocytes, glutamate increased ObRa and ObRb expression, whereas leptin pretreatment attenuated glial cytotoxicity by excess glutamate, reflected by better preserved adenosine triphosphate production. The protective role of astrocytic leptin signaling is further supported by the higher lethality of the astrocyte-specific leptin receptor knockout mice in the initial phase of seizure production. Thus, leptin signaling in astrocytes plays a protective role against seizure, and the effects are at least partially mediated by attenuation of glutamate toxicity. Astrocytic leptin signaling, therefore, may be a novel therapeutic target.

Leptin regulates glutamate and glucose transporters in hypothalamic astrocytes

Glial cells perform critical functions that alter the metabolism and activity of neurons, and there is increasing interest in their role in appetite and energy balance. Leptin, a key regulator of appetite and metabolism, has previously been reported to influence glial structural proteins and morphology. Here, we demonstrate that metabolic status and leptin also modify astrocyte-specific glutamate and glucose transporters, indicating that metabolic signals influence synaptic efficacy and glucose uptake and, ultimately, neuronal function. We found that basal and glucose-stimulated electrical activity of hypothalamic proopiomelanocortin (POMC) neurons in mice were altered in the offspring of mothers fed a high-fat diet. In adulthood, increased body weight and fasting also altered the expression of glucose and glutamate transporters. These results demonstrate that whole-organism metabolism alters hypothalamic glial cell activity and suggest that these cells play an important role in the pathology of obesity.

Emerging role of glial cells in the control of body weight

Glia are the most abundant cell type in the brain and are indispensible for the normal execution of neuronal actions. They protect neurons from noxious insults and modulate synaptic transmission through affectation of synaptic inputs, release of glial transmitters and uptake of neurotransmitters from the synaptic cleft. They also transport nutrients and other circulating factors into the brain thus controlling the energy sources and signals reaching neurons. Moreover, glia express receptors for metabolic hormones, such as leptin and insulin, and can be activated in response to increased weight gain and dietary challenges. However, chronic glial activation can be detrimental to neurons, with hypothalamic astrocyte activation or gliosis suggested to be involved in the perpetuation of obesity and the onset of secondary complications. It is now accepted that glia may be a very important participant in metabolic control and a possible therapeutical target. Here we briefly review this rapidly advancing field.

Intranasal delivery of insulin via the olfactory nerve pathway

OBJECTIVES

Intranasal delivery has been shown to target peptide therapeutics to the central nervous system (CNS) of animal models and induce specific neurological responses. In an investigation into the pathways by which intranasal administration delivers insulin to the CNS, this study has focused on the direct delivery of insulin from the olfactory mucosa to the olfactory bulbs via the olfactory nerve pathway.

METHODS

Nasal and olfactory tissues of mice were imaged with fluorescent and electron microscopy 30 min following intranasal administration.

KEY FINDINGS

Macroscopic analysis confirmed delivery to the anterior regions of the olfactory bulbs. Confocal microscopy captured delivery along the olfactory nerve bundles exiting the nasal mucosa, traversing the cribriform plate and entering the bulbs. With electron microscopy, insulin was found within cells of the olfactory nerve layer and glomerular layer of the olfactory bulbs.

CONCLUSIONS

These results demonstrated that intranasal administration of labelled insulin targeted the CNS through the olfactory nerve pathway in mice.

Upregulation of astrocytic leptin receptor in mice with experimental autoimmune encephalomyelitis

The detrimental role of leptin in experimental autoimmune encephalomyelitis (EAE) is opposite to its neuroprotective role in other neuropathologies. We hypothesize that a shifted cellular distribution of leptin receptors underlies the differential effects of leptin. A robust increase of ObR immunoreactivity was seen along glial fibrillary acidic protein (GFAP)(+) intermediate filaments in reactive astrocytes in the hippocampus and hypothalamus of mice with EAE. Although astrocyte-specific GFAP mRNA and protein were both increased, ObRa mRNA was elevated only after resolution of EAE symptoms, and ObRb mRNA was even decreased at the peak time of symptoms of EAE. A cell type-specific action of leptin may underlie its differential effects.

Leptin action on nonneuronal cells in the CNS: potential clinical applications

Leptin, an adipocyte-derived cytokine, crosses the blood–brain barrier to act on many regions of the central nervous system (CNS). It participates in the regulation of energy balance, inflammatory processes, immune regulation, synaptic formation, memory condensation, and neurotrophic activities. This review focuses on the newly identified actions of leptin on astrocytes. We first summarize the distribution of leptin receptors in the brain, with a focus on the hypothalamus, where the leptin receptor is known to mediate essential feeding suppression activities, and on the hippocampus, where leptin facilitates memory, reduces neurodegeneration, and plays a dual role in seizures. We will then discuss regulation of the nonneuronal leptin system in obesity. Its relationship with neuronal leptin signaling is illustrated by in vitro assays in primary astrocyte culture and by in vivo studies on mice after pretreatment with a glial metabolic inhibitor or after cell-specific deletion of intracellular signaling leptin receptors. Overall, the glial leptin system shows robust regulation and plays an essential role in obesity. Strategies to manipulate this nonneuronal leptin signaling may have major clinical impact.

Endothelial leptin receptor mutation provides partial resistance to diet-induced obesity

Leptin, a polypeptide hormone produced mainly by adipocytes, has diverse effects in both the brain and peripheral organs, including suppression of feeding. Other than mediating leptin transport across the blood-brain barrier, the role of the endothelial leptin receptor remains unclear. We recently generated a mutant mouse strain lacking endothelial leptin receptor signaling, and showed that there is an increased uptake of leptin by brain parenchyma after its delivery by in situ brain perfusion. Here, we tested the hypothesis that endothelial leptin receptor mutation confers partial resistance to diet-induced obesity. These ELKO mice had similar body weight and percent fat as their wild-type littermates when fed with rodent chow, but blood concentrations of leptin were significantly elevated. In response to a high-fat diet, wild-type mice had a greater gain of body weight and fat than ELKO mice. As shown by metabolic chamber measurement, the ELKO mice had higher oxygen consumption, carbon dioxide production, and heat dissipation, although food intake was similar to that of the wild-type mice and locomotor activity was even reduced. This indicates that the partial resistance to diet-induced obesity was mediated by higher metabolic activity in the ELKO mice. Since neuronal leptin receptor knockout mice show obesity and diabetes, the results suggest that endothelial leptin signaling shows opposite effects from that of neuronal leptin signaling, with a facilitatory role in diet-induced obesity.

Effects of cell-type specific leptin receptor mutation on leptin transport across the BBB

The functions of leptin receptors (LRs) are cell-type specific. At the blood-brain barrier, LRs mediate leptin transport that is essential for its CNS actions, and both endothelial and astrocytic LRs may be involved. To test this, we generated endothelia specific LR knockout (ELKO) and astrocyte specific LR knockout (ALKO) mice. ELKO mice were derived from a cross of Tie2-cre recombinase mice with LR-floxed mice, whereas ALKO mice were generated by a cross of GFAP-cre with LR-floxed mice, yielding mutant transmembrane LRs without signaling functions in endothelial cells and astrocytes, respectively. The ELKO mutation did not affect leptin half-life in blood or apparent influx rate to the brain and spinal cord, though there was an increase of brain parenchymal uptake of leptin after in situ brain perfusion. Similarly, the ALKO mutation did not affect blood-brain barrier permeation of leptin or its degradation in blood and brain. The results support our observation from cellular studies that membrane-bound truncated LRs are fully efficient in transporting leptin, and that basal levels of astrocytic LRs do not affect leptin transport across the endothelial monolayer. Nonetheless, the absence of leptin signaling at the BBB appears to enhance the availability of leptin to CNS parenchyma. The ELKO and ALKO mice provide new models to determine the dynamic regulation of leptin transport in metabolic and inflammatory disorders where cellular distribution of LRs is shifted.