Astrocytic insulin receptor controls circadian behavior via dopamine signaling in a sexually dimorphic manner

Mammalian circadian clocks respond to feeding and light cues, adjusting internal rhythms with day/night cycles. Astrocytes serve as circadian timekeepers, driving daily physiological rhythms; however, it's unknown how they ensure precise cycle-to-cycle rhythmicity. This is critical for understanding why mistimed or erratic feeding, as in shift work, disrupts circadian physiology- a condition linked to type 2 diabetes and obesity. Here, we show that astrocytic insulin signaling sets the free-running period of locomotor activity in female mice and food entrainment in male mice. Additionally, ablating the insulin receptor in hypothalamic astrocytes alters cyclic energy homeostasis differently in male and female mice. Remarkably, the mutants exhibit altered dopamine metabolism, and the pharmacological modulation of dopaminergic signaling partially restores distinct circadian traits in both male and female mutant mice. Our findings highlight the role of astrocytic insulin-dopaminergic signaling in conveying time-of-feeding or lighting cues to the astrocyte clock, thus governing circadian behavior in a sex-specific manner.

High-calorie diets uncouple hypothalamic oxytocin neurons from a gut-to-brain satiation pathway via κ-opioid signaling

Oxytocin-expressing paraventricular hypothalamic neurons (PVNOT neurons) integrate afferent signals from the gut, including cholecystokinin (CCK), to adjust whole-body energy homeostasis. However, the molecular underpinnings by which PVNOT neurons orchestrate gut-to-brain feeding control remain unclear. Here, we show that mice undergoing selective ablation of PVNOT neurons fail to reduce food intake in response to CCK and develop hyperphagic obesity on a chow diet. Notably, exposing wild-type mice to a high-fat/high-sugar (HFHS) diet recapitulates this insensitivity toward CCK, which is linked to diet-induced transcriptional and electrophysiological aberrations specifically in PVNOT neurons. Restoring OT pathways in diet-induced obese (DIO) mice via chemogenetics or polypharmacology sufficiently re-establishes CCK's anorexigenic effects. Last, by single-cell profiling, we identify a specialized PVNOT neuronal subpopulation with increased κ-opioid signaling under an HFHS diet, which restrains their CCK-evoked activation. In sum, we document a (patho)mechanism by which PVNOT signaling uncouples a gut-brain satiation pathway under obesogenic conditions.

Estradiol regulates leptin sensitivity to control feeding via hypothalamic Cited1

Until menopause, women have a lower propensity to develop metabolic diseases than men, suggestive of a protective role for sex hormones. Although a functional synergy between central actions of estrogens and leptin has been demonstrated to protect against metabolic disturbances, the underlying cellular and molecular mechanisms mediating this crosstalk have remained elusive. By using a series of embryonic, adult-onset, and tissue/cell-specific loss-of-function mouse models, we document an unprecedented role of hypothalamic Cbp/P300-interacting transactivator with Glu/Asp-rich carboxy-terminal domain 1 (Cited1) in mediating estradiol (E2)-dependent leptin actions that control feeding specifically in pro-opiomelanocortin (Pomc) neurons. We reveal that within arcuate Pomc neurons, Cited1 drives leptin's anorectic effects by acting as a co-factor converging E2 and leptin signaling via direct Cited1-ERα-Stat3 interactions. Together, these results provide new insights on how melanocortin neurons integrate endocrine inputs from gonadal and adipose axes via Cited1, thereby contributing to the sexual dimorphism in diet-induced obesity.

Hypothalamic astrocytes control systemic glucose metabolism and energy balance

The hypothalamus is key in the control of energy balance. However, strategies targeting hypothalamic neurons have failed to provide viable options to treat most metabolic diseases. Conversely, the role of astrocytes in systemic metabolic control has remained largely unexplored. Here, we show that obesity promotes anatomically restricted remodeling of hypothalamic astrocyte activity. In the paraventricular nucleus (PVN) of the hypothalamus, chemogenetic manipulation of astrocytes results in bidirectional control of neighboring neuron activity, autonomic outflow, glucose metabolism, and energy balance. This process recruits a mechanism involving the astrocytic control of ambient glutamate levels, which becomes defective in obesity. Positive or negative chemogenetic manipulation of PVN astrocyte Ca2+ signals, respectively, worsens or improves metabolic status of diet-induced obese mice. Collectively, these findings highlight a yet unappreciated role for astrocytes in the direct control of systemic metabolism and suggest potential targets for anti-obesity strategy.

Diet triggers specific responses of hypothalamic astrocytes in time and region dependent manner

Hypothalamic astrocytes are particularly affected by energy-dense food consumption. How the anatomical location of these glial cells and their spatial molecular distribution in the arcuate nucleus of the hypothalamus (ARC) determine the cellular response to a high caloric diet remains unclear. In this study, we investigated their distinctive molecular responses following exposure to a high-fat high-sugar (HFHS) diet, specifically in the ARC. Using RNA sequencing and proteomics, we showed that astrocytes have a distinct transcriptomic and proteomic profile dependent on their anatomical location, with a major proteomic reprogramming in hypothalamic astrocytes. By ARC single-cell sequencing, we observed that a HFHS diet dictates time- and cell- specific transcriptomic responses, revealing that astrocytes have the most distinct regulatory pattern compared to other cell types. Lastly, we topographically and molecularly characterized astrocytes expressing glial fibrillary acidic protein and/or aldehyde dehydrogenase 1 family member L1 in the ARC, of which the abundance was significantly increased, as well as the alteration in their spatial and molecular profiles, with a HFHS diet. Together, our results provide a detailed multi-omics view on the spatial and temporal changes of astrocytes particularly in the ARC during different time points of adaptation to a high calorie diet.

Synaptotagmin-13 Is a Neuroendocrine Marker in Brain, Intestine and Pancreas

Synaptotagmin-13 (Syt13) is an atypical member of the vesicle trafficking synaptotagmin protein family. The expression pattern and the biological function of this Ca²⁺-independent protein are not well resolved. Here, we have generated a novel Syt13-Venus fusion (Syt13-VF) fluorescence reporter allele to track and isolate tissues and cells expressing Syt13 protein. The reporter allele is regulated by endogenous cis-regulatory elements of Syt13 and the fusion protein follows an identical expression pattern of the endogenous Syt13 protein. The homozygous reporter mice are viable and fertile. We identify the expression of the Syt13-VF reporter in different regions of the brain with high expression in tyrosine hydroxylase (TH)-expressing and oxytocin-producing neuroendocrine cells. Moreover, Syt13-VF is highly restricted to all enteroendocrine cells in the adult intestine that can be traced in live imaging. Finally, Syt13-VF protein is expressed in the pancreatic endocrine lineage, allowing their specific isolation by flow sorting. These findings demonstrate high expression levels of Syt13 in the endocrine lineages in three major organs harboring these secretory cells. Collectively, the Syt13-VF reporter mouse line provides a unique and reliable tool to dissect the spatio-temporal expression pattern of Syt13 and enables isolation of Syt13-expressing cells that will aid in deciphering the molecular functions of this protein in the neuroendocrine system.

Small extracellular vesicle-mediated targeting of hypothalamic AMPKα1 corrects obesity through BAT activation

Current pharmacological therapies for treating obesity are of limited efficacy. Genetic ablation or loss of function of AMP-activated protein kinase alpha 1 (AMPKα1) in steroidogenic factor 1 (SF1) neurons of the ventromedial nucleus of the hypothalamus (VMH) induces feeding-independent resistance to obesity due to sympathetic activation of brown adipose tissue (BAT) thermogenesis. Here, we show that body weight of obese mice can be reduced by intravenous injection of small extracellular vesicles (sEVs) delivering a plasmid encoding an AMPKα1 dominant negative mutant (AMPKα1-DN) targeted to VMH-SF1 neurons. The beneficial effect of SF1-AMPKα1-DN-loaded sEVs is feeding-independent and involves sympathetic nerve activation and increased UCP1-dependent thermogenesis in BAT. Our results underscore the potential of sEVs to specifically target AMPK in hypothalamic neurons and introduce a broader strategy to manipulate body weight and reduce obesity.

Insulin action on astrocytes: From energy homeostasis to behaviour

Astrocytes are specialised glial cells that integrate distinct inputs arising from neurones, other glial cells and the microcirculation to regulate diverse aspects of brain function. A growing body of emerging evidence supports that astrocytes, similar to neurones, also play active roles in the neuroendocrine control of metabolism by responding to afferent nutritional and hormonal cues and translating these metabolic cues into neuronal inputs. Specifically, insulin action in astrocytes has received special emphasis given its newly discovered regulatory role in brain glucose uptake, which until recently was assumed to be an insulin independent process. We now know that insulin signalling in astrocytes regulates metabolic processes and behavioural responses through coupling brain glucose uptake with nutrient availability to maintain energy balance and systemic glucose homeostasis. Moreover, genetic ablation of the insulin receptor in astrocytes is associated with anxiety- and depressive-like behaviours, confirming that these glial cells are involved in the regulation of cognition and mood via insulin action. Here, we provide a comprehensive review of the most relevant findings that have been made over the course of the last few years linking insulin signalling in astrocytes with the pathogenesis of brain metabolic and neurodegenerative diseases; a still unexplored field, but with a high translational potential for developing therapies.

Active integrins regulate white adipose tissue insulin sensitivity and brown fat thermogenesis

Objective

Reorganization of the extracellular matrix is a prerequisite for healthy adipose tissue expansion, whereas fibrosis is a key feature of adipose dysfunction and inflammation. However, very little is known about the direct effects of impaired cell–matrix interaction in adipocyte function and insulin sensitivity. The objective of this study was to determine whether integrin activity can regulate insulin sensitivity in adipocytes and thereby systemic metabolism.

Methods

We characterized integrin activity in adipose tissue and its consequences on whole-body metabolism using adipose-selective deletion of β1 integrin (Itgb1^adipo-cre) and Kindlin-2 (Kind2^adipo-cre) in mice.

Results

We demonstrate that integrin signaling regulates white adipocyte insulin action and systemic metabolism. Consequently, loss of adipose integrin activity, similar to loss of adipose insulin receptors, results in a lipodystrophy-like phenotype and systemic insulin resistance. However, brown adipose tissue of Kind2^adipo-cre and Itgb1^adipo-cre mice is chronically hyperactivated and has increased substrate delivery, reduced endothelial basement membrane thickness, and increased endothelial vesicular transport.

Conclusions

Thus, we establish integrin-extracellular matrix interactions as key regulators of white and brown adipose tissue function and whole-body metabolism.

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β1 and β3 integrins interact with insulin signaling to regulate white adipocyte insulin sensitivity and systemic metabolism.

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Impaired integrin activity results in lipodystrophy in the absence of hepatosteatosis.

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β1 integrin activity regulates energy expenditure and vascular permeability in brown adipose tissue.

Identification and characterization of distinct brown adipocyte subtypes in C57BL/6J mice

Using a number of cell biological and statistical methods we identify and characterize EIF5, TCF25 and BIN1 as markers for individual brown adipocyte subtypes in C57BL/6J mice.

Brown adipose tissue (BAT) plays an important role in the regulation of body weight and glucose homeostasis. Although increasing evidence supports white adipose tissue heterogeneity, little is known about heterogeneity within murine BAT. Recently, UCP1 high and low expressing brown adipocytes were identified, but a developmental origin of these subtypes has not been studied. To obtain more insights into brown preadipocyte heterogeneity, we use single-cell RNA sequencing of the BAT stromal vascular fraction of C57/BL6 mice and characterize brown preadipocyte and adipocyte clonal cell lines. Statistical analysis of gene expression profiles from brown preadipocyte and adipocyte clones identify markers distinguishing brown adipocyte subtypes. We confirm the presence of distinct brown adipocyte populations in vivo using the markers EIF5, TCF25, and BIN1. We also demonstrate that loss of Bin1 enhances UCP1 expression and mitochondrial respiration, suggesting that BIN1 marks dormant brown adipocytes. The existence of multiple brown adipocyte subtypes suggests distinct functional properties of BAT depending on its cellular composition, with potentially distinct functions in thermogenesis and the regulation of whole body energy homeostasis.

OR33-06 Potential Role of Mutations in TBX3 in Human Weight Regulation

Introduction:Tbx3 has been shown to play a role in the terminal specification of hypothalamic melanocortin neurons during neonatal development & in maintaining the plasticity of their peptidergic role in adulthood in animal experiments (1). The absence of humans with biallelic mutations in TBX3 & the conservation of the critical domains across species emphasizes its essential role in life. Heterozygous mutations in humans have been associated with ulnar mammary syndrome (UMS) with a spectrum of phenotype including obesity. Based on these observations, we hypothesized that heterozygous mutations in the conserved regions of TBX3 may play an important role in the weight regulation pathway in humans.

Methods: The Genetics of Early Childhood Obesity (GECO) study enrolls children with severe (BMI > 120% of 95^th %tile of CDC reference) early onset (< 6 years) obesity. Whole exome sequencing (WES) was performed in a subset of proband-parent trios. Peripheral mononuclear cells (PBMCs) from selected families were collected to generate induced pluripotent stem cells using non-integrating Sendai virus. Differentiation into disease-relevant hypothalamic neurons was performed using the published protocol (2). Loss-of-function models & isogenic controls were created using CRISPR/Cas9 & their cellular/molecular phenotypes were obtained at several time points during the course of differentiation.

Results: We have identified a family with heterozygous mutation in TBX3 (p.His205Tyr, c.613 C>T, g.115118728 G>A). The proband is an 11-year old boy with morbid obesity (BMI 43.9 kg/m², BMIz + 3.25), advanced bone age, precocious puberty & type 2 diabetes. Trio analysis ruled out recessive & compound heterozygote causative variants, & none of the identified de novo variants were considered pathogenic. His mother suffers from severe obesity (BMI 38 kg/m² post-bariatric surgery) supporting an autosomal dominant inheritance of the phenotype. The putative causal variant in TBX3 segregates in the proband, mother & maternal grandmother. Located in the DNA binding domain of T-box, the variant is predicted to be deleterious by 4 in silico algorithms & rare in population-based databases (mean allele frequency 0.006% in gNOMAD, absent in ClinVar). Consistent with the variable penetrance of the phenotype in UMS, neither mother nor child have the classic features, but, the mother has uterine anomalies causing 6 spontaneous abortions & was unable to breast feed due to inverted nipples. Ongoing functional studies in human hypothalamic neurons suggest that a decrease in melanocortin signaling possibly explaining the phenotype in this family.

Conclusions: Mutations in TBX3 in humans may have a role as a monogenic cause of obesity and disease-relevant hypothalamic stem cells can serve as models to study them.

Ref: 1) Quarta et al. Nat Metab 2019, 1(2), 222-35; 2) Wang et al. JCI, 125(2): 796-808

Functional identity of hypothalamic melanocortin neurons depends on Tbx3

Heterogeneous populations of hypothalamic neurons orchestrate energy balance via the release of specific signatures of neuropeptides. However, how specific intracellular machinery controls peptidergic identities and function of individual hypothalamic neurons remains largely unknown. The transcription factor T-box 3 (Tbx3) is expressed in hypothalamic neurons sensing and governing energy status, whereas human TBX3 haploinsufficiency has been linked with obesity. Here, we demonstrate that loss of Tbx3 function in hypothalamic neurons causes weight gain and other metabolic disturbances by disrupting both the peptidergic identity and plasticity of Pomc/Cart and Agrp/Npy neurons. These alterations are observed after loss of Tbx3 in both immature hypothalamic neurons and terminally differentiated mouse neurons. We further establish the importance of Tbx3 for body weight regulation in Drosophila melanogaster and show that TBX3 is implicated in the differentiation of human embryonic stem cells into hypothalamic Pomc neurons. Our data indicate that Tbx3 directs the terminal specification of neurons as functional components of the melanocortin system and is required for maintaining their peptidergic identity. In summary, we report the discovery of a key mechanistic process underlying the functional heterogeneity of hypothalamic neurons governing body weight and systemic metabolism.

Streptozotocin-induced β-cell damage, high fat diet, and metformin administration regulate Hes3 expression in the adult mouse brain

Diabetes mellitus is a group of disorders characterized by prolonged high levels of circulating blood glucose. Type 1 diabetes is caused by decreased insulin production in the pancreas whereas type 2 diabetes may develop due to obesity and lack of exercise; it begins with insulin resistance whereby cells fail to respond properly to insulin and it may also progress to decreased insulin levels. The brain is an important target for insulin, and there is great interest in understanding how diabetes affects the brain. In addition to the direct effects of insulin on the brain, diabetes may also impact the brain through modulation of the inflammatory system. Here we investigate how perturbation of circulating insulin levels affects the expression of Hes3, a transcription factor expressed in neural stem and progenitor cells that is involved in tissue regeneration. Our data show that streptozotocin-induced β-cell damage, high fat diet, as well as metformin, a common type 2 diabetes medication, regulate Hes3 levels in the brain. This work suggests that Hes3 is a valuable biomarker helping to monitor the state of endogenous neural stem and progenitor cells in the context of diabetes mellitus.

Lipoprotein Lipase Maintains Microglial Innate Immunity in Obesity

Consumption of a hypercaloric diet upregulates microglial innate immune reactivity along with a higher expression of lipoprotein lipase (Lpl) within the reactive microglia in the mouse brain. Here, we show that knockdown of the Lpl gene specifically in microglia resulted in deficient microglial uptake of lipid, mitochondrial fuel utilization shifting to glutamine, and significantly decreased immune reactivity. Mice with knockdown of the Lpl gene in microglia gained more body weight than control mice on a high-carbohydrate high-fat (HCHF) diet. In these mice, microglial reactivity was significantly decreased in the mediobasal hypothalamus, accompanied by downregulation of phagocytic capacity and increased mitochondrial dysmorphologies. Furthermore, HCHF-diet-induced POMC neuronal loss was accelerated. These results show that LPL-governed microglial immunometabolism is essential to maintain microglial function upon exposure to an HCHF diet. In a hypercaloric environment, lack of such an adaptive immunometabolic response has detrimental effects on CNS regulation of energy metabolism.

Molecular Integration of Incretin and Glucocorticoid Action Reverses Immunometabolic Dysfunction and Obesity

Chronic inflammation has been proposed to contribute to the pathogenesis of diet-induced obesity. However, scarce therapeutic options are available to treat obesity and the associated immunometabolic complications. Glucocorticoids are routinely employed for the management of inflammatory diseases, but their pleiotropic nature leads to detrimental metabolic side effects. We developed a glucagon-like peptide-1 (GLP-1)-dexamethasone co-agonist in which GLP-1 selectively delivers dexamethasone to GLP-1 receptor-expressing cells. GLP-1-dexamethasone lowers body weight up to 25% in obese mice by targeting the hypothalamic control of feeding and by increasing energy expenditure. This strategy reverses hypothalamic and systemic inflammation while improving glucose tolerance and insulin sensitivity. The selective preference for GLP-1 receptor bypasses deleterious effects of dexamethasone on glucose handling, bone integrity, and hypothalamus-pituitary-adrenal axis activity. Thus, GLP-1-directed glucocorticoid pharmacology represents a safe and efficacious therapy option for diet-induced immunometabolic derangements and the resulting obesity.

Dietary sugars, not lipids, drive hypothalamic inflammation

Objective

The hypothalamus of hypercaloric diet-induced obese animals is featured by a significant increase of microglial reactivity and its associated cytokine production. However, the role of dietary components, in particular fat and carbohydrate, with respect to the hypothalamic inflammatory response and the consequent impact on hypothalamic control of energy homeostasis is yet not clear.

Methods

We dissected the different effects of high-carbohydrate high-fat (HCHF) diets and low-carbohydrate high-fat (LCHF) diets on hypothalamic inflammatory responses in neurons and non-neuronal cells and tested the hypothesis that HCHF diets induce hypothalamic inflammation via advanced glycation end-products (AGEs) using mice lacking advanced glycation end-products (AGEs) receptor (RAGE) and/or the activated leukocyte cell-adhesion molecule (ALCAM).

Results

We found that consumption of HCHF diets, but not of LCHF diets, increases microgliosis as well as the presence of N(ε)-(Carboxymethyl)-Lysine (CML), a major AGE, in POMC and NPY neurons of the arcuate nucleus. Neuron-secreted CML binds to both RAGE and ALCAM, which are expressed on endothelial cells, microglia, and pericytes. On a HCHF diet, mice lacking the RAGE and ALCAM genes displayed less microglial reactivity and less neovasculature formation in the hypothalamic ARC, and this was associated with significant improvements of metabolic disorders induced by the HCHF diet.

Conclusions

Combined overconsumption of fat and sugar, but not the overconsumption of fat per se, leads to excessive CML production in hypothalamic neurons, which, in turn, stimulates hypothalamic inflammatory responses such as microgliosis and eventually leads to neuronal dysfunction in the control of energy metabolism.

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HCHF, but not LCHF diets, induce obesity and increase the hypothalamic inflammatory response.

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A HCHF diet increases N-epsilon-(carboxymethyl)lysine content in hypothalamic neurons in the ARC.

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Obesity and metabolic symptoms induced by a HCHF diet are improved in mice lacking functional RAGE and ALCAM genes.

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Lacking RAGE and ALCAM prevents the hypothalamic inflammatory response and angiogenesis that occur on a HCHF diet.

TNFα drives mitochondrial stress in POMC neurons in obesity

Consuming a calorically dense diet stimulates microglial reactivity in the mediobasal hypothalamus (MBH) in association with decreased number of appetite-curbing pro-opiomelanocortin (POMC) neurons; whether the reduction in POMC neuronal function is secondary to the microglial activation is unclear. Here we show that in hypercaloric diet-induced obese mice, persistently activated microglia in the MBH hypersecrete TNFα that in turn stimulate mitochondrial ATP production in POMC neurons, promoting mitochondrial fusion in their neurites, and increasing POMC neuronal firing rates and excitability. Specific disruption of the gene expressions of TNFα downstream signals TNFSF11A or NDUFAB1 in the MBH of diet-induced obese mice reverses mitochondrial elongation and reduces obesity. These data imply that in a hypercaloric environment, persistent elevation of microglial reactivity and consequent TNFα secretion induces mitochondrial stress in POMC neurons that contributes to the development of obesity.

Astrocytic Insulin Signaling Couples Brain Glucose Uptake with Nutrient Availability

We report that astrocytic insulin signaling co-regulates hypothalamic glucose sensing and systemic glucose metabolism. Postnatal ablation of insulin receptors (IRs) in glial fibrillary acidic protein (GFAP)-expressing cells affects hypothalamic astrocyte morphology, mitochondrial function, and circuit connectivity. Accordingly, astrocytic IR ablation reduces glucose-induced activation of hypothalamic pro-opio-melanocortin (POMC) neurons and impairs physiological responses to changes in glucose availability. Hypothalamus-specific knockout of astrocytic IRs, as well as postnatal ablation by targeting glutamate aspartate transporter (GLAST)-expressing cells, replicates such alterations. A normal response to altering directly CNS glucose levels in mice lacking astrocytic IRs indicates a role in glucose transport across the blood-brain barrier (BBB). This was confirmed in vivo in GFAP-IR KO mice by using positron emission tomography and glucose monitoring in cerebral spinal fluid. We conclude that insulin signaling in hypothalamic astrocytes co-controls CNS glucose sensing and systemic glucose metabolism via regulation of glucose uptake across the BBB.

The opposing effects of ghrelin on hypothalamic and systemic inflammatory processes are modulated by its acylation status and food intake in male rats

Ghrelin is an endogenous hormone that stimulates appetite and adipose tissue accrual. Both the acylated (AG) and non-acylated (DAG) isoforms of this hormone are also reported to exert anti-inflammatory and protective effects systemically and in the central nervous system. As inflammatory processes have been implicated in obesity-associated secondary complications, we hypothesized that this natural appetite stimulator may protect against negative consequences resulting from excessive food intake. Adult male Wistar rats were treated icv (5 μg/day) with AG, DAG, the ghrelin mimetic GH-releasing peptide (GHRP)-6, AG, and pair-fed with controls (AG-pf) or saline for 14 days. Regardless of food intake AG increased visceral adipose tissue (VAT) and decreased circulating cytokine levels. However, AG reduced cytokine production in VAT only in rats fed ad libitum. Hypothalamic cytokine production was increased in AG-treated rats fed ad libitum and by DAG, but intracellular inflammatory signaling pathways associated with insulin and leptin resistance were unaffected. Gliosis was not observed in response to any treatment as glial markers were either reduced or unaffected. AG, DAG, and GHRP-6 stimulated production of hypothalamic insulin like-growth factor I that is involved in cell protective mechanisms. In hypothalamic astrocyte cell cultures AG decreased tumor necrosis factorα and DAG decreased interleukin-1β mRNA levels, suggesting direct anti-inflammatory effects on astrocytes. Thus, whereas ghrelin stimulates food intake and weight gain, it may also induce mechanisms of cell protection that help to detour or delay systemic inflammatory responses and hypothalamic gliosis due to excess weight gain, as well as its associated pathologies.

The metabolic response to postnatal leptin in rats varies with age and may be litter dependent

Hyperleptinemia during postnatal life induces long-term effects on metabolism. However, these effects are controversial as both increased and decreased propensity towards obesity has been reported. To further analyze the effects of chronic neonatal hyperleptinemia on the subsequent metabolic profile, male Wistar rats proceeding from 18 different litters (8 pups/litter) received a daily subcutaneous injection of either saline (10 ml/kg, n=36) or leptin (3 μg/g, n=36) from postnatal day (PND) 2 to PND9. Rats were sacrificed at 10, 40, or 150 days of age. At 10 days of age, leptin treated rats had decreased body weight (p<0.001) and body fat (p<0.05). Leptin levels and glycemia were increased (p<0.01), whereas insulin, total lipids, triglycerides and glycerol levels were decreased (p<0.05). At PND40 rats receiving leptin had increased glycemia (p<0.01) and plasma HDL and LDL levels, but decreased total lipids (p<0.05). At PND150 neonatal leptin treatment induced different effects in rats raised in different litters. Rats from litter 1 had increased body weight (p<0.05), body fat (p<0.01), and plasma leptin (p<0.001), cholesterol (p<0.001), triglyceride (p<0.001), total lipid (p<0.001), LDL (p<0.05), and glycerol (p<0.001) levels. In rats from litter 2 these parameters did not differ from controls. Rats from litter 3 had decreased body weight (p<0.05), visceral fat (p<0.01) and plasma leptin (p<0.001), cholesterol (p<0.001), triglyceride (p<0.001), glycerol (p<0.001), and HDL (p<0.001) levels. In conclusion, the metabolic response to postnatal leptin varies with age, with the response in adulthood being variable and most likely influenced by other factors, including the genetic make-up.

The emerging neurobiology of calorie addiction

The response of the brain to sugar is determined by specific cell populations in the brain, including neurons that secrete melanin-concentrating hormone, and culminates in the release of dopamine.

Hormones and diet, but not body weight, control hypothalamic microglial activity

The arcuate nucleus (ARC) of the hypothalamus plays a key role in sensing metabolic feedback and regulating energy homeostasis. Recent studies revealed activation of microglia in mice with high-fat diet (HFD)-induced obesity (DIO), suggesting a potential pathophysiological role for inflammatory processes within the hypothalamus. To further investigate the metabolic causes and molecular underpinnings of such glial activation, we analyzed the microglial activity in wild-type (WT), monogenic obese ob/ob (leptin deficient), db/db (leptin-receptor mutation), and Type-4 melanocortin receptor knockout (MC4R KO) mice on either a HFD or on standardized chow (SC) diet. Following HFD exposure, we observed a significant increase in the total number of ARC microglia, immunoreactivity of ionized calcium binding adaptor molecule 1 (iba1-ir), cluster of differentiation 68 (CD68-ir), and ramification of microglial processes. The ob/ob mice had significantly less iba1-ir and ramifications. Leptin replacement rescued these phenomena. The db/db mice had similar iba1-ir comparable with WT mice but had significantly lower CD68-ir and more ramifications than WT mice. After 2 weeks of HFD, ob/ob mice showed an increase of iba1-ir, and db/db mice showed increase of CD68-ir. Obese MC4R KO mice fed a SC diet had comparable iba1-ir and CD68-ir with WT mice but had significantly more ramifications than WT mice. Intriguingly, treatment of DIO mice with glucagon-like peptide-1 receptor agonists reduced microglial activation independent of body weight. Our results show that diet type, adipokines, and gut signals, but not body weight, affect the presence and activity levels of hypothalamic microglia in obesity.

Hypothalamic inflammation without astrogliosis in response to high sucrose intake is modulated by neonatal nutrition in male rats

Hypothalamic inflammation and gliosis are proposed to participate in the pathogenesis of high-fat diet-induced obesity. Because other factors and nutrients also induce weight gain and adiposity, we analyzed the inflammatory and glial responses to a sucrose (S)-enriched diet. Neonatal overnutrition (NON) exacerbates weight gain in response to metabolic challenges; thus, we compared the inflammatory response of male Wistar rats with NON (4 pups/litter) and controls (12 pups/litter) to increased S intake. At weaning rats received water or a 33% sucrose solution and normal chow ad libitum for 2 months. Sucrose increased serum IL-1β and -6 and hypothalamic IL-6 mRNA levels in NON and TNFα mRNA levels in control and NON rats, whereas NON alone had no effect. The astrocyte marker glial fibrillary acidic protein was increased by NON but decreased by S. This was associated with hypothalamic nuclei specific changes in glial fibrillary acidic protein-positive cell number and morphology. Sucrose increased the number of microglia and phosphorylation of inhibitor of -κB and c-Jun N-terminal kinase in control but not NON rats, with no effect on microglia activation markers. Proteins highly expressed in astrocytes (glutamate, glucose, and lactate transporters) were increased by NON but not S, with no increase in vimentin expression in astrocytes, further suggesting that S-induced adiposity is not associated with hypothalamic astrogliosis. Hence, activation of hypothalamic inflammatory processes and gliosis depend not only on weight gain but also on the diet inducing this weight gain and the early nutritional status. These diverse inflammatory processes could indicate a differential disposition to obesity-induced pathologies.

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.

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.

Early postnatal overnutrition increases adipose tissue accrual in response to a sucrose-enriched diet

Both overnutrition and an incorrect nutrient balance have contributed to the rise in obesity. Moreover, it is now clear that poor nutrition during early life augments the possibility of excess weight gain in later years. Our aim was to determine how neonatal overnutrition affects later responses to a sucrose-enriched diet and whether this varies depending upon when the diet is introduced in postnatal life. Male Wistar rats raised in litters of four or 12 pups were given a 33% sucrose solution instead of water from weaning (day 21) or postnatal day (PND) 65. All rats received normal chow ad libitum until they were euthanized on PND 80. Body weight (BW) and food and liquid intake were monitored throughout the study. Fat mass, adipocyte morphology, serum biochemical and hormonal parameters, and hypothalamic neuropeptide mRNA levels were measured at study termination. Neonatal overnutrition increased food intake, BW, and leptin levels, induced adipocyte hypertrophy, and decreased total ghrelin levels. The sucrose-enriched diet increased total energy intake, adipose accrual, and leptin, adiponectin, and acylated ghrelin levels but decreased BW. Most of these responses were accentuated in neonatally overnourished rats, which also had increased insulin and triglyceride levels. However, long-term sucrose intake induced adipocyte hypertrophy in rats from normal-sized litters but not in neonatally overfed rats. The results reported here indicate that neonatal overnutrition increases the detrimental response to a diet rich in sucrose later in life. Moreover, the timing and duration of the exposure to a sucrose-enriched diet alter the adverse metabolic outcomes.

Early nutritional changes induce sexually dimorphic long-term effects on body weight gain and the response to sucrose intake in adult rats

Long-term metabolic effects induced by early nutritional changes are suspected to differ between males and females, but few studies have analyzed both sexes simultaneously. We analyzed the consequences of neonatal nutritional changes on body weight (BW) and the adult response to a sucrose-enriched diet in both male and female rats. Litter size was manipulated at birth to induce over- and undernutrition (4 pups: L4; 12 pups: L12; 20 pups: L20). From 50 to 65 days of age, half of each group received a 33% sucrose solution instead of water. Serum leptin, insulin, and ghrelin levels were analyzed at day 65. At weaning, rats from L4 weighed more and those from L20 weighed less than controls (L12). Body weight was greater in L4 rats throughout the study and increased further compared with controls in adult life. L20 males ate less and gained less weight throughout the study, but L20 females had a significant catch-up in BW. Sucrose intake increased total energy consumption in all groups, but not BW gain, with L4 males and L4 and L20 females reducing weight gain. Yet, sucrose intake increased serum leptin levels, with this increase being significant in L4 and L20 males. Our results suggest that females are more capable than males of recuperating and maintaining a normal BW after reduced neonatal nutrition. Furthermore, increased sucrose intake does not increase BW, but could alter body composition as reflected by leptin levels, with the percentage of calories consumed in the form of sucrose being affected by sex and neonatal nutrition.

Leptin in early life: a key factor for the development of the adult metabolic profile

Leptin levels during the perinatal period are important for the development of metabolic systems involved in energy homeostasis. In rodents, there is a postnatal leptin surge, with circulating leptin levels increasing around postnatal day (PND) 5 and peaking between PND 9 and PND 10. At this time circulating leptin acts as an important trophic factor for the development of hypothalamic circuits that control energy homeostasis and food seeking and reward behaviors. Blunting the postnatal leptin surge results in long-term leptin insensitivity and increased susceptibility to diet-induced obesity during adulthood. Pharmacologically increased leptin levels in the postnatal period also have long-term effects on metabolism. Nevertheless, this effect is controversial as postnatal hyperleptinemia is reported to both increase and decrease the predisposition to obesity in adulthood. The different effects reported in the literature could be explained by the different moments at which this hormone was administered, suggesting that modifications of the neonatal leptin surge at specific time points could selectively affect the development of central and peripheral systems that are undergoing modifications at this moment resulting in different metabolic and behavioral outcomes. In addition, maternal nutrition and the hormonal environment during pregnancy and lactation may also modulate the offspring's response to postnatal modifications in leptin levels. This review highlights the importance of leptin levels during the perinatal period in the development of metabolic systems that control energy homeostasis and how modifications of these levels may induce long-lasting and potentially irreversible effects on metabolism.

Prenatal stress induces long-term effects in cell turnover in the hippocampus-hypothalamus-pituitary axis in adult male rats

Subchronic gestational stress leads to permanent modifications in the hippocampus-hypothalamus-pituitary-adrenal axis of offspring probably due to the increase in circulating glucocorticoids known to affect prenatal programming. The aim of this study was to investigate whether cell turnover is affected in the hippocampus-hypothalamus-pituitary axis by subchronic prenatal stress and the intracellular mechanisms involved. Restraint stress was performed in pregnant rats during the last week of gestation (45 minutes; 3 times/day). Only male offspring were used for this study and were sacrificed at 6 months of age. In prenatally stressed adults a decrease in markers of cell death and proliferation was observed in the hippocampus, hypothalamus and pituitary. This was associated with an increase in insulin-like growth factor-I mRNA levels, phosphorylation of CREB and calpastatin levels and inhibition of calpain -2 and caspase -8 activation. Levels of the anti-apoptotic protein Bcl-2 were increased and levels of the pro-apoptotic factor p53 were reduced. In conclusion, prenatal restraint stress induces a long-term decrease in cell turnover in the hippocampus-hypothalamus-pituitary axis that might be at least partly mediated by an autocrine-paracrine IGF-I effect. These changes could condition the response of this axis to future physiological and pathophysiological situations.

Effects of acute changes in neonatal leptin levels on food intake and long-term metabolic profiles in rats

In rodents there is a rise in serum leptin levels between postnatal days (PND) 5 and 14, with this neonatal leptin surge reported to modulate the maturation of hypothalamic circuits involved in appetite regulation. We hypothesized that acute changes in neonatal leptin levels have different long-term metabolic effects depending on how and when this surge is modified. To advance the timing of the normal leptin peak, male Wistar rats were injected with leptin (sc, 3 μg/g) on PND 2. To ablate the leptin peak on PND 10, a pegylated leptin antagonist (sc, 9 μg/g) was injected. Controls received vehicle. All rats were allowed to eat ad libitum until PND 150. Increased leptin on PND 2 reduced food intake (P<0.01) after 3 months of age with no effect on body weight. Levels of total ghrelin were reduced (P<0.001) and acylated ghrelin increased (P<0.05), with no other modifications in metabolic hormones. In contrast, treatment with the leptin antagonist on PND 9 did not affect food intake but reduced body weight beginning around PND 60 (P<0.02). This was associated with a reduction in fat mass, insulin (P<0.01), and leptin (P<0.007) levels and an increase in testosterone levels (P<0.01). Hypothalamic neuropeptide Y (P<0.05) and leptin receptor (P<0.005) mRNA levels were reduced, whereas mRNA levels for uncoupling protein 2 (P<0.005) were increased in visceral fat, which may indicate an increase in energy expenditure. In conclusion, acute changes in neonatal leptin levels induce different metabolic profiles depending on how and when leptin levels are modified.

Differential acute and chronic effects of leptin on hypothalamic astrocyte morphology and synaptic protein levels

Astrocytes participate in neuroendocrine functions partially through modulation of synaptic input density in the hypothalamus. Indeed, glial ensheathing of neurons is modified by specific hormones, thus determining the availability of neuronal membrane space for synaptic inputs, with the loss of this plasticity possibly being involved in pathological processes. Leptin modulates synaptic inputs in the hypothalamus, but whether astrocytes participate in this action is unknown. Here we report that astrocyte structural proteins, such as glial fibrillary acidic protein (GFAP) and vimentin, are induced and astrocyte morphology modified by chronic leptin administration (intracerebroventricular, 2 wk), with these changes being inversely related to modifications in synaptic protein densities. Similar changes in glial structural proteins were observed in adult male rats that had increased body weight and circulating leptin levels due to neonatal overnutrition (overnutrition: four pups/litter vs. control: 12 pups/litter). However, acute leptin treatment reduced hypothalamic GFAP levels and induced synaptic protein levels 1 h after administration, with no effect on vimentin. In primary hypothalamic astrocyte cultures leptin also reduced GFAP levels at 1 h, with an induction at 24 h, indicating a possible direct effect of leptin. Hence, one mechanism by which leptin may affect metabolism is by modifying hypothalamic astrocyte morphology, which in turn could alter synaptic inputs to hypothalamic neurons. Furthermore, the responses to acute and chronic leptin exposure are inverse, raising the possibility that increased glial activation in response to chronic leptin exposure could be involved in central leptin resistance.

Insulin and growth hormone-releasing peptide-6 (GHRP-6) have differential beneficial effects on cell turnover in the pituitary, hypothalamus and cerebellum of streptozotocin (STZ)-induced diabetic rats

Poorly controlled type1 diabetes is associated with hormonal imbalances and increased cell death in different tissues, including the pituitary, hypothalamus and cerebellum. In the pituitary, lactotrophs are the cell population with the greatest increase in cell death, whereas in the hypothalamus and cerebellum astrocytes are most highly affected. Insulin treatment can delay, but does not prevent, diabetic complications. As ghrelin and growth hormone (GH) secretagogues are reported to prevent apoptosis in different tissues, and to modulate glucose homeostasis, a combined hormonal treatment may be beneficial. Hence, we analyzed the effect of insulin and GH-releasing peptide 6 (GHRP-6) on diabetes-induced apoptosis in the pituitary, hypothalamus and cerebellum of diabetic rats. Adult male Wistar rats were made diabetic by streptozotocin injection (65 mg/kg ip) and divided into four groups from diabetes onset: those receiving a daily sc injection of saline (1 ml/kg/day), GHRP-6 (150 μg/kg/day), insulin (1-8U/day) or insulin plus GHRP-6 for 8 weeks. Control non-diabetic rats received saline (1 ml/kg/day). Diabetes increased cell death in the pituitary, hypothalamus and cerebellum (P<0.05). In the pituitary, insulin treatment prevented diabetes-induced apoptosis (P<0.01), as well as the decline in prolactin and GH mRNA levels (P<0.05). In the hypothalamus, neither insulin nor GHRP-6 decreased diabetes-induced cell death. However, the combined treatment of insulin+GHRP-6 prevented the diabetes induced-decrease in glial fibrillary acidic protein (GFAP) levels (P<0.05). In the cerebellum, although insulin treatment increased GFAP levels (P<0.01), only the combined treatment of insulin+ GHRP-6 decreased diabetes-induced apoptosis (P<0.05). In conclusion, insulin and GHRP-6 exert tissue specific effects in STZ-diabetic rats and act synergistically on some processes. Indeed, insulin treatment does not seem to be effective on preventing some of the diabetes-induced alterations in the central nervous system.

Activation of microglia in specific hypothalamic nuclei and the cerebellum of adult rats exposed to neonatal overnutrition

Much attention has been drawn to the possible involvement of hypothalamic inflammation in the pathogenesis of metabolic disorders, especially in response to a high-fat diet. Microglia, the macrophages of the central nervous system, can be activated by proinflammatory signals resulting in the local production of specific interleukins and cytokines, which in turn could exacerbate the pathogenic process. Because obesity itself is considered to be a state of chronic inflammation, we evaluated whether being overweight results in microglial activation in the hypothalamus of rats on a normal diet. Accordingly, we used a model of neonatal overnutrition that entailed adjustment of litter size at birth (small litters: four pups/dam versus normal litters: 12 pups/dam) and resulted in a 15% increase in bodyweight and increased circulating leptin levels at postnatal day 60. Rats that were overnourished during neonatal life had an increased number of activated microglia in specific hypothalamic areas such as the ventromedial hypothalamus, which is an important site for metabolic control. However, this effect was not confined to the hypothalamus because significant microglial activation was also observed in the cerebellar white matter. There was no change in circulating tumour necrosis factor (TNF) α levels or TNFα mRNA levels in either the hypothalamus or cerebellum. Interleukin (IL)6 protein levels were higher in both the hypothalamus and cerebellum, with no change in IL6 mRNA levels. Because circulating IL6 levels were elevated, this rise in central IL6 could be a result of increased uptake. Thus, activation of microglia occurs in adult rats exposed to neonatal overnutrition and a moderate increase in weight gain on a normal diet, possibly representing a secondary response to systemic inflammation. Moreover, this activation could result in local changes in specific hypothalamic nuclei that in turn further deregulate metabolic homeostasis.

Gender differences in the long-term effects of chronic prenatal stress on the HPA axis and hypothalamic structure in rats

Stress during pregnancy can impair biological and behavioral responses in the adult offspring and some of these effects are associated with structural changes in specific brain regions. Furthermore, these outcomes can vary according to strain, gender, and type and duration of the maternal stress. Indeed, early stress can induce sexually dimorphic long-term effects on diverse endocrine axes, including subsequent responses to stress. However, whether hypothalamic structural modifications are associated with these endocrine disruptions has not been reported. Thus, we examined the gender differences in the long-term effects of prenatal and adult immobilization stress on the hypothalamic-pituitary-adrenocortical (HPA) axis and the associated changes in hypothalamic structural proteins. Pregnant Wistar rats were subjected to immobilization stress three times daily (45 min each) during the last week of gestation. One half of the offspring were subjected to the same regimen of stress on 10 consecutive days starting at postnatal day (PND) 90. At sacrifice (PND 180), serum corticosterone levels were significantly higher in females compared to males and increased significantly in females subjected to both stresses with no change in males. Prenatal stress increased pituitary ACTH content in males, with no effect in females. Hypothalamic CRH mRNA levels were significantly increased by prenatal stress in females, but decreased in male rats. In females neither stress affected hypothalamic cell death, as determined by cytoplasmic histone-associated DNA fragment levels or proliferation, determined by proliferating cell nuclear antigen levels (PCNA); however, in males there was a significant decrease in cell death in response to prenatal stress and a decrease in PCNA levels with both prenatal and adult stress. In all groups BrdU immunoreactivity colocalized in glial fibrillary acidic protein (GFAP) positive cells, with few BrdU/NeuN labelled cells found. Furthermore, in males the astrocyte marker S100β increased with prenatal stress and decreased with adult stress, suggesting affectation of astrocytes. Synapsin-1 levels were increased by adult stress in females and by prenatal stress in males, while, PSD95 levels were increased in females and decreased in males by both prenatal and adult stress. In conclusion, hypothalamic structural rearrangement appears to be involved in the long-term endocrine outcomes observed after both chronic prenatal and adult stresses. Furthermore, many of these changes are not only different between males and females, but opposite, which could underlie the gender differences in the long-term sequelae of chronic stress, including subsequent responses to stress.

The positive effects of growth hormone-releasing peptide-6 on weight gain and fat mass accrual depend on the insulin/glucose status

Ghrelin and GH secretagogues, including GH-releasing peptide (GHRP)-6, stimulate food intake and adiposity. Because insulin modulates the hypothalamic response to GH secretagogues and acts synergistically with ghrelin on lipogenesis in vitro, we analyzed whether insulin plays a role in the metabolic effects of GHRP-6 in vivo. Streptozotocin-induced diabetic rats received saline, GHRP-6, insulin, or insulin plus GHRP-6 once daily for 8 wk. Rats receiving saline suffered hyperglycemia, hyperphagia, polydipsia, and weight loss. Insulin, but not GHRP-6, improved these parameters (P < 0.001 for all), as well as the diabetes-induced increase in hypothalamic mRNA levels of neuropeptide Y and agouti-related peptide and decrease in proopiomelanocortin. Cocaine amphetamine-related transcript mRNA levels were also reduced in diabetic rats, with GHRP-6 inducing a further decrease (P < 0.03) and insulin an increase. Diabetic rats receiving insulin plus GHRP-6 gained more weight and had increased epididymal fat mass and serum leptin levels compared with all other groups (P < 0.001). In epididymal adipose tissue, diabetic rats injected with saline had smaller adipocytes (P < 0.001), decreased fatty acid synthase (FAS; P < 0.001), and glucose transporter-4 (P < 0.001) and increased hormone sensitive lipase (P < 0.001) and proliferator-activated receptor-gamma mRNA levels (P < 0.01). Insulin normalized these parameters to control values. GHRP-6 treatment increased FAS and glucose transporter-4 gene expression and potentiated insulin's effect on epididymal fat mass, adipocyte size (P < 0.001), FAS (P < 0.001), and glucose transporter-4 (P < 0.05). In conclusion, GHRP-6 and insulin exert an additive effect on weight gain and visceral fat mass accrual in diabetic rats, indicating that some of GHRP-6's metabolic effects depend on the insulin/glucose status.

The weight gain response to stress during adulthood is conditioned by both sex and prenatal stress exposure

Food intake and weight gain are known to be affected by stress. However, the type and duration of the stress may have variable effects, with males and females responding differently. We report the short-term and long-term effects of prenatal and adult immobilization stress, as well as the combination of these two stresses, on weight gain and food intake in male and female rats and the role of post-pubertal gonadal hormones in this process. No long-term effect of prenatal stress on food intake or weight gain was found in either sex. However, during the period of adult stress [at postnatal day (P) 90; 10 days duration] stressed male rats gained significantly less weight than controls and previous exposure to prenatal stress attenuated this effect (control: 31.2+/-2.1g; prenatal stress: 24.6+/-3.8g; adult stress: 8.1+/-3.4g; prenatal and adult stress: 18.2+/-3.3g; p<0.0001). There was no change in food intake in response to either prenatal or adult stress. Adult stress increased circulating corticosterone levels during the initial part of the stress period, in both male and female rats with this rise being greater in male rats. No effect on corticosterone levels was observed on the last day of stress in either sex. No effect on weight gain or food intake was observed in female rats. Following adult stress, male rats increased their weight gain, with no change in food intake, such that 1 month later they reached control levels. At the time of sacrifice (P180), there were no differences in weight or circulating metabolic hormone levels between any of the male groups. Although castration alone modulated body weight in both male and female rats, it did not affect their weight gain response to adult stress. These results indicate that the weight gain response to adult stress is sexually dimorphic and that this is not dependent on post-pubertal gonadal steroids. Furthermore, the outcome of this response closely depends on the time at which the change in weight is analyzed, which could help to explain different results reported in the literature. Indeed, weight and metabolic hormone levels were normalized by the end of the study.

Ghrelin treatment protects lactotrophs from apoptosis in the pituitary of diabetic rats

Poorly controlled diabetes is associated with hormonal imbalances, including decreased prolactin production partially due to increased lactotroph apoptosis. In addition to its metabolic actions, ghrelin inhibits apoptosis in several cell types. Thus, we analyzed ghrelin's effects on diabetes-induced pituitary cell death and hormonal changes. Six weeks after onset of diabetes in male Wistar rats (streptozotocin 70 mg/kg), minipumps infusing saline or 24 nmol ghrelin/day were implanted (jugular). Rats were killed two weeks later. Ghrelin did not modify body weight or serum glucose, leptin or adiponectin, but increased total ghrelin (P<0.05), IGF-I (P<0.01) and prolactin (P<0.01) levels. Ghrelin decreased cell death, iNOS and active caspase-8 (P<0.05) and increased prolactin (P<0.05), Bcl-2 (P<0.01) and Hsp70 (P<0.05) content in the pituitary. In conclusion, ghrelin prevents diabetes-induced death of lactotrophs, decreasing caspase-8 activation and iNOS content and increasing anti-apoptotic pathways such as pituitary Bcl-2 and Hsp70 and serum IGF-I concentrations.

Death of hypothalamic astrocytes in poorly controlled diabetic rats is associated with nuclear translocation of apoptosis inducing factor

Astrocytes in the hypothalamus of poorly controlled diabetic rats are reduced in number, due to increased apoptosis and decreased proliferation, and undergo morphological changes, including a decrease in projections. These changes are associated with modifications in synaptic proteins and most likely affect neuroendocrine signalling and function. The present study aimed to determine the intracellular mechanisms underlying this increase in hypothalamic cell death. Adult male Wistar rats were injected with streptozotocin (70 mg/kg, i.p) and controls received vehicle. Rats were killed at 1, 4, 6 and 8 weeks after diabetes onset (glycaemia > 300 mg/dl). Cell death, as detected by enzyme-linked immunosorbent assay, increased at 4 weeks of diabetes. Immunohistochemistry and terminal dUTP nick-end labelling (TUNEL) assays indicated that these cells corresponded to glial fibrillary acidic protein (GFAP) positive cells. No significant change in fragmentation of caspases 2, 3, 6, 7, 8, 9, or 12 was observed with western blot analysis. However, enzymatic assays indicated that caspase 3 activity increased significantly after 1 week of diabetes and decreased below control levels thereafter. In the hypothalamus, cell bodies lining the third ventricle, fibres radiating from the third ventricle and GFAP positive cells expressed fragmented caspase 3, with this labelling increasing at 1 week of diabetes. However, because no nuclear labelling was observed and this increase in activity did not correlate temporally with the increased cell death, this caspase may not be involved in astrocyte death. By contrast, nuclear translocation of apoptosis inducing factor (AIF) increased significantly in astrocytes in parallel with the increase in death and AIF was found in TUNEL positive cells. Thus, nuclear translocation of AIF could underlie the increased death, whereas fragmentation of caspase 3 could be associated with the morphological changes found in hypothalamic astrocytes of diabetic rats.

Reduction in the number of astrocytes and their projections is associated with increased synaptic protein density in the hypothalamus of poorly controlled diabetic rats

Processes under hypothalamic control, such as thermogenesis, feeding behavior, and pituitary hormone secretion, are disrupted in poorly controlled diabetes, but the underlying mechanisms are poorly understood. Because glial cells regulate neurosecretory neurons through modulation of synaptic inputs and function, we investigated the changes in hypothalamic glia in rats with streptozotocin-induced diabetes mellitus. Hypothalamic glial fibrillary acidic protein (GFAP) levels decreased significantly 6 wk after diabetes onset. This was coincident with decreased GFAP immunoreactive surface area, astrocyte number, and the extension of GFAP immunoreactive processes/astrocyte in the arcuate nucleus. Cell death, analyzed by terminal deoxyuridine 5-triphosphate nick-end labeling and ELISA, increased significantly at 4 wk of diabetes. Proliferation, measured by Western blot for proliferating cell nuclear antigen and immunostaining for phosphorylated histone H-3, decreased in the hypothalamus of diabetic rats throughout the study, becoming significantly reduced by 8 wk. Both proliferation and death affected astroctyes because both phosphorylated histone H-3- and terminal deoxyuridine 5-triphosphate nick-end labeling-labeled cells were GFAP positive. Western blot analysis revealed that postsynaptic density protein 95 and the presynaptic proteins synapsin I and synaptotagmin increased significantly at 8 wk of diabetes, suggesting increased hypothalamic synaptic density. Thus, in poorly controlled diabetic rats, there is a decrease in the number of hypothalamic astrocytes that is correlated with modifications in synaptic proteins and possibly synaptic inputs. These morphological changes in the arcuate nucleus could be involved in neurosecretory and metabolic changes seen in diabetic animals.

Activation of the intrinsic cell death pathway, increased apoptosis and modulation of astrocytes in the cerebellum of diabetic rats

Poorly controlled diabetes mellitus results in structural and functional changes in many brain regions. We demonstrate that in streptozotocin-induced diabetic rats cell death is increased and proliferation decreased in the cerebellum, indicating overall cell loss. Levels of both the proform and cleaved forms of caspases 3, 6 and 9 are increased, with no change in caspases 7, 8 or 12. Colocalization of glial fibrillary acidic protein (GFAP) and cleaved caspase 3 and GFAP in TUNEL-positive cells increased in diabetic rats. Changes in GFAP levels paralleled modifications in proliferating cell nuclear antigen (PCNA), increasing at 1 week of diabetes and decreasing thereafter, and proliferating GFAP-positive cells were decreased in the cerebellum of diabetic rats. These results suggest that astrocytes are dramatically affected in the cerebellum, including an increase in cell death and a decrease in proliferation, and this could play a role in the structural and functional changes in this brain area in diabetes.