Lipopolysaccharide-induced leptin release is neurally controlled

Adipokines as Diagnostic and Prognostic Markers for the Severity of COVID-19

Accumulating evidence implicates obesity as a risk factor for increased severity of disease outcomes in patients infected with severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2). Obesity is associated with adipose tissue dysfunction, which not only predisposes individuals to metabolic complications, but also substantially contributes to low-grade systemic inflammation, altered immune cell composition, and compromised immune function. This seems to impact the susceptibility and outcome of diseases caused by viruses, as obese people appear more vulnerable to developing infections and they recover later from infectious diseases than normal-weight individuals. Based on these findings, increased efforts to identify suitable diagnostic and prognostic markers in obese Coronavirus disease 2019 (COVID-19) patients to predict disease outcomes have been made. This includes the analysis of cytokines secreted from adipose tissues (adipokines), which have multiple regulatory functions in the body; for instance, modulating insulin sensitivity, blood pressure, lipid metabolism, appetite, and fertility. Most relevant in the context of viral infections, adipokines also influence the immune cell number, with consequences for overall immune cell activity and function. Hence, the analysis of the circulating levels of diverse adipokines in patients infected with SARS-CoV-2 have been considered to reveal diagnostic and prognostic COVID-19 markers. This review article summarizes the findings aimed to correlate the circulating levels of adipokines with progression and disease outcomes of COVID-19. Several studies provided insights on chemerin, adiponectin, leptin, resistin, and galectin-3 levels in SARS-CoV-2-infected patients, while limited information is yet available on the adipokines apelin and visfatin in COVID-19. Altogether, current evidence points at circulating galectin-3 and resistin levels being of diagnostic and prognostic value in COVID-19 disease.

Chronic Leptin Deficiency Improves Tolerance of Physiological Damage and Host-Pathogen Cooperation during Yersinia pseudotuberculosis Infection

To combat infections, hosts employ a combination of antagonistic and cooperative defense strategies. The former refers to pathogen killing mediated by resistance mechanisms, while the latter refers to physiological defense mechanisms that promote host health during infection independent of pathogen killing, leading to an apparent cooperation between the host and the pathogen. Previous work has shown that Leptin, a pleiotropic hormone that plays a central role in regulating appetite and energy metabolism, is indispensable for resistance mechanisms, while a role for Leptin signaling in cooperative host-pathogen interactions remains unknown. Using a mouse model of Yersinia pseudotuberculosis (Yptb) infection, an emerging pathogen that causes fever, diarrhea, and mesenteric lymphadenitis in humans, we found that the physiological effects of chronic Leptin-signaling deficiency conferred protection from Yptb infection due to increased host-pathogen cooperation rather than greater resistance defenses. The protection against Yptb infection was independent of differences in food consumption, lipolysis, or fat mass. Instead, we found that the chronic absence of Leptin signaling protects from a shift to lipid utilization during infection that contributes to Yptb lethality. Furthermore, we found that the survival advantage conferred by Leptin deficiency was associated with increased liver and kidney damage. Our work reveals an additional level of complexity for the role of Leptin in infection defense and demonstrates that in some contexts, in addition to tolerating the pathogen, tolerating organ damage is more beneficial for survival than preventing the damage.

Lipopolysaccharide-Induced Strain-Specific Differences in Neuroinflammation and MHC-I Pathway Regulation in the Brains of Bl6 and 129Sv Mice

Many studies have demonstrated significant mouse-strain-specific differences in behavior and response to pathogenic and pharmacological agents. This study seeks to characterize possible differences in microglia activation and overall severity of neuroinflammation in two widely used mouse strains, C57BL/6NTac (Bl6) and 129S6/SvEvTac (129Sv), in response to acute lipopolysaccharide (LPS) administration. Locomotor activity within the open field arena revealed similar 24 h motor activity decline in both strains. Both strains also exhibited significant bodyweight loss due to LPS treatment, although it was more severe in the Bl6 strain. Furthermore, LPS induced a hypothermic response in Bl6 mice, which was not seen in 129Sv. We found that 24 h LPS challenge significantly increased the inflammatory status of microglia in 129Sv mice. On the other hand, we observed that, under physiological conditions, microglia of Bl6 seemed to be in a higher immune-alert state. Gene and protein expression analysis revealed that LPS induces a significantly stronger upregulation of MHC-I-pathway-related components in the brain of Bl6 compared to 129Sv mice. The most striking difference was detected in the olfactory bulb, where we observed significant LPS-induced upregulation of MHC-I pathway components in Bl6 mice, whereas no alterations were observed in 129Sv. We observed significant positive correlations between bodyweight decline and expressions of MHC-I components in the olfactory bulbs of Bl6 mice and the frontal cortex of 129Sv, highlighting different brain regions most affected by LPS in these strains. Our findings suggest that the brains of Bl6 mice exist in a more immunocompetent state compared to 129Sv mice.

Leptin and the Blood-Brain Barrier: Curiosities and Controversies

Leptin for over 25 years has been a central theme in the study of appetite, obesity, and starvation. As the major site of leptin production is peripheral, and the site of action of greatest interest is the hypothalamus, how leptin accesses the central nervous system (CNS) and crosses the blood-brain barrier (BBB) has been of great interest. We review here the ongoing research that addresses fundamental questions such as the sites of leptin resistances in obesity and other conditions, the causes of resistances and their relations to one another, the three barrier sites of entry into the CNS, why recent studies using suprapharmacological doses cannot address these questions but give insight into nonsaturable entry of leptin into the CNS, and how that might be useful in using leptin therapeutically. The current status of the controversy of whether the short form of the leptin receptor acts as the BBB leptin transporter and how obesity may transform leptin transport is reviewed. Review of these and other topics summarizes in a new appreciation of what leptin may have actually evolved to do and what physiological role leptin resistance may play. © 2021 American Physiological Society. Compr Physiol 11:1-19, 2021.

Associations Among Maternal Metabolic Conditions, Cord Serum Leptin Levels, and Autistic Symptoms in Children

INTRODUCTION

Accumulating evidence has shown that maternal metabolic conditions, such as pre-pregnancy overweight, diabetes mellitus, and hypertensive disorders of pregnancy (HDP) are potential risk factors of autism spectrum disorder (ASD). However, it remains unclear how these maternal conditions lead to neurodevelopmental outcomes in the offspring, including autistic symptoms. Leptin, an adipokine that has pro-inflammatory effects and affects fetal neurodevelopment, is a candidate mediator of the association between maternal metabolic factors and an increased risk of ASD. However, whether prenatal exposure to leptin mediates the association between maternal metabolic conditions and autistic symptoms in children has not been investigated yet.

METHODS

This study investigated the associations between mothers' metabolic conditions (pre-pregnancy overweight, diabetes mellitus during or before pregnancy, and HDP), leptin concentrations in umbilical cord serum, and autistic symptoms among 762 children from an ongoing cohort study, using generalized structural equation modeling. We used the Social Responsive Scale, Second Edition (SRS-2) at 8-9 years old to calculate total T-scores. Additionally, we used the T-scores for two subdomains: Social Communication and Interaction (SCI) and Restricted Interests and Repetitive Behavior (RRB).

RESULTS

Umbilical cord leptin levels were associated with pre-pregnancy overweight [coefficient = 1.297, 95% confidence interval (CI) 1.081-1.556, p = 0.005] and diabetes mellitus (coefficient = 1.574, 95% CI 1.206-2.055, p = 0.001). Furthermore, leptin levels were significantly associated with SRS-2 total T-scores (coefficient = 1.002, 95% CI 1.000-1.004, p = 0.023), SCI scores (coefficient = 1.002, 95% CI 1.000-1.004, p = 0.020), and RRB scores (coefficient = 1.001, 95% CI 1.000-1.003, p = 0.044) in children. Associations between maternal metabolic factors and autistic symptoms were not significant.

DISCUSSION

The present study uncovered an association between cord leptin levels and autistic symptoms in children, while maternal metabolic conditions did not have an evident direct influence on the outcome. These results imply that prenatal pro-inflammatory environments affected by maternal metabolic conditions may contribute to the development of autistic symptoms in children. The findings warrant further investigation into the role of leptin in the development of autistic symptoms.

Influence of peripheral lipopolysaccharide (LPS) on feed intake, body temperature and hypothalamic expression of neuropeptides involved in appetite regulation in broilers and layer chicks

1. This study examined the expression of genes related to appetite-regulating neuropeptides in the hypothalamus of broiler and layer chicks (Gallus gallus) after intraperitoneal (IP) injection of lipopolysaccharide (LPS). 2. Both broiler and layer chicks received (n = 10 per group) LPS at doses of 0 and 200 µg and feed intake was measured up to 6 h after injection. In a further experiment, (n = 8 per group) mRNA abundance of some hypothalamic neuropeptides was measured 2 h after injection. The rectal temperature of each chick was measured before and 2 h post-injection. 3. Feed intake was significantly decreased by LPS from 2 h after injection and thereafter, while the rectal temperature did not change. 4. LPS decreased the expression of appetite-enhancing neuropeptides: neuropeptide Y (NPY) and agouti-related peptide (AgRP) in broilers and, NPY in layer chicks. The expression of appetite-suppressing neuropeptides (corticotrophin-releasing factor (CRF), proopiomelanocortin (POMC) and, cocaine and amphetamine regulated-transcript (CART) was not changed in broilers, while CRF tended to decrease and POMC was significantly decreased in layers. The abundance of the cytokine tumour necrosis factor-alpha (TNF-α) did not change in broilers but was decreased in layers. 5. The findings indicated that the reduction in gene expression of hypothalamic appetite-enhancing neuropeptides NPY and AgRP is responsible for anorexia caused by LPS at a dose that did not influence body temperature.

Age-Dependent Changes of Adipokine and Cytokine Secretion From Rat Adipose Tissue by Endogenous and Exogenous Toll-Like Receptor Agonists

White adipose tissue but recently also brown adipose tissue have emerged as endocrine organs. Age-associated obesity is accompanied by prolonged and elevated lipopolysaccharide (LPS)-induced sickness symptoms and increased cytokine and adipokine levels in the circulation partially originating from adipose tissue. In the present study, ex vivo fat explants were used to investigate how the exogenous pathogen-associated molecular pattern (PAMP) LPS or the endogenous danger-associated molecular patterns (DAMPs) high mobility group box-1 protein (HMGB1) and biglycan modulate the release of cytokines and adipokines/batokines and, thus, could influence systemic and/or local inflammation. The response of adipose tissue (epididymal, retroperitoneal, subcutaneous, and brown) was compared between young lean and old obese rats (2 vs. 24 months old). LPS induced a strong interleukin (IL)-6 and tumor necrosis factor (TNF) alpha release into the supernatant of all adipose tissue types investigated. HMGB1 (subcutaneous) and biglycan (retroperitoneal) led to an increased release of IL-6 and TNFalpha (HMGB1) and decreased visfatin and adiponectin (biglycan) secretion from epididymal adipose tissue (young rats). Visfatin was also decreased by HMGB1 in retroperitoneal adipose tissue of old rats. We found significantly higher leptin (all fat pads) and adiponectin (subcutaneous) levels in supernatants of adipose tissue from old compared to young rats, whereas visfatin secretion showed the opposite. The expression of the biglycan receptor Toll-like receptor (TLR) 2 as well as the LPS and HMGB1 receptors TLR4 and receptor for advanced glycation end products (RAGE) were reduced with age (TLR4/RAGE) and by stimulation with their ligands (subcutaneous). Overall, we revealed that adipokines/adipose-tissue released cytokines show some modulation of their release caused by mediators of septic (batokines) and sterile inflammation with potential implication for acute and chronic disease. Moreover, aging may increase or decrease the release of fat-derived mediators. These data show that DAMPS and LPS locally modulate cytokine secretion while only DAMPS but not LPS can locally alter adipokine secretion during inflammation.

Maternal Immune Activation and the Development of Dopaminergic Neurotransmission of the Offspring: Relevance for Schizophrenia and Other Psychoses

Prenatal infections have been linked to the development of schizophrenia (SCZ) and other neurodevelopmental disorders in the offspring, and work in animal models indicates that this is to occur through the maternal inflammatory response triggered by infection. Several studies in animal models demonstrated that acute inflammatory episodes are sufficient to trigger brain alterations in the adult offspring, especially in the mesolimbic dopamine (DA) system, involved in the pathophysiology of SCZ and other disorders involving psychosis. In the current review, we synthesize the literature on the clinical studies implicating prenatal infectious events in the development of SCZ. Then, we summarize evidence from animal models of maternal immune activation (MIA) and the behavioral and molecular alterations relevant for the function of the DAergic system. Furthermore, we discuss the evidence supporting the involvement of maternal cytokines, such as interleukin 6 (IL-6) and leptin (a hormone with effects on inflammation) in mediating the effects of MIA on the fetal brain, leading to the long-lasting effects on the offspring. In particular, IL-6 has been involved in mediating the effects of MIA animal models in the offspring through actions on the placenta, induction of IL-17a, or triggering the decrease in non-heme iron (hypoferremia). Maternal infection is very likely interacting with additional genetic and environmental risk factors in the development of SCZ; systematically investigating how these interactions produce specific phenotypes is the next step in understanding the etiology of complex psychiatric disorders.

Elucidating the role of leptin in systemic inflammation: a study targeting physiological leptin levels in rats and their macrophages

To elucidate the role of leptin in acute systemic inflammation, we investigated how its infusion at low, physiologically relevant doses affects the responses to bacterial lipopolysaccharide (LPS) in rats subjected to 24 h of food deprivation. Leptin was infused subcutaneously (0–20 μg·kg−1·h−1) or intracerebroventricularly (0–1 μg·kg−1·h−1). Using hypothermia and hypotension as biomarkers of systemic inflammation, we identified the phase extending from 90 to 240 min post-LPS as the most susceptible to modulation by leptin. In this phase, leptin suppressed the rise in plasma TNF-α and accelerated the recoveries from hypothermia and hypotension. Suppression of TNF-α was not accompanied by changes in other cytokines or prostaglandins. Leptin suppressed TNF-α when infused peripherally but not when infused into the brain. Importantly, the leptin dose that suppressed TNF-α corresponded to the lowest dose that limited food consumption; this dose elevated plasma leptin within the physiological range (to 5.9 ng/ml). We then conducted in vitro experiments to investigate whether an action of leptin on macrophages could parallel our in vivo observations. The results revealed that, when sensitized by food deprivation, LPS-stimulated peritoneal macrophages can be inhibited by leptin at concentrations that are lower than those reported to promote cytokine release. It is concluded that physiological levels of leptin do not exert a proinflammatory effect but rather an anti-inflammatory effect involving selective suppression of TNF-α via an action outside the brain. The mechanism of this effect might involve a previously unrecognized, suppressive action of leptin on macrophage subpopulations sensitized by food deprivation, but future studies are warranted.

The development of depression-like behavior is consolidated by IL-6-induced activation of locus coeruleus neurons and IL-1β-induced elevated leptin levels in mice

RATIONALE

Many studies have supported the cytokine hypothesis as the underlying pathophysiology of depressive disorder.

OBJECTIVES

We previously reported that lipopolysaccharide (LPS)-induced depression-like behavior is abrogated by the α1-adrenoceptor antagonist prazosin. Since cytokines are involved in LPS effects on the brain, we investigated the effects of cytokines on noradrenergic neurons in the locus coeruleus (LC) and whether central α1-adrenoceptors can cause the development of depression-like behavior.

METHODS

Adult male CD1 mice were treated with LPS (1 mg/kg, i.p.) or saline and sacrificed 2 h later for immunofluorescence studies of c-fos and tyrosine hydroxylase (TH) expression in LC neurons. Serum cytokines were measured using enzyme-linked immunosorbent assay (ELISA). Another group of mice were implanted with intracerebroventricular (i.c.v.) cannulae and given artificial cerebrospinal fluid (CSF) (control), interleukin (IL)-1β (0.5 μg), IL-6 (1 μg), or tumor necrosis factor (TNF)-α (1 μg), and sacrificed 2 h later for c-fos and TH immunofluorescence analysis. Serum samples were analyzed for leptin levels. In addition, tail suspension test (TST), forced swimming test (FST), and sucrose preference (SP) test were conducted in a separate group of mice treated i.c.v. with cytokines, recombinant mouse leptin (5 μg) or phenylephrine (40 μg). These effects were countered by i.c.v. administration of prazosin and a leptin antagonist.

RESULTS

LPS increased c-fos expression in TH-positive neurons. Central administration of IL-6 and IL-1β increased c-fos immunoreactivity and serum leptin levels. Phenylephrine, an α1-adrenoceptor agonist, given i.c.v., increased the immobility time during FST and decreased SP, but had no effect on TST. Central leptin administration increased immobility time during FST but did not affect TST or SP. The combination of phenylephrine and leptin increased immobility time during FST and TST, and decreased SP. Induction of depression-like behavior by co-administration of IL-1β and IL-6 was prevented by pretreatment with prazosin alone.

CONCLUSION

These results suggest that IL-6-dependent LC neuronal activation induced depression-like behavior and IL-1β-induced increase in leptin levels enhanced α1-adrenoceptor-mediated depression-like behavior.

Obesity, adipokines and neuroinflammation

Global levels of obesity are reaching epidemic proportions, leading to a dramatic increase in incidence of secondary diseases and the significant economic burden associated with their treatment. These comorbidities include diabetes, cardiovascular disease, and some psychopathologies, which have been linked to a low-grade inflammatory state. Obese individuals exhibit an increase in circulating inflammatory mediators implicated as the underlying cause of these comorbidities. A number of these molecules are also manufactured and released by white adipose tissue (WAT), in direct proportion to tissue mass and are collectively known as adipokines. In the current review we focused on the role of two of the better-studied members of this family namely, leptin and adiponectin, with particular emphasis on their role in neuro-immune interactions, neuroinflammation and subsequent brain diseases. This article is part of a Special Issue entitled 'Neuroimmunology and Synaptic Function'.

Temporal gene expression in the hippocampus and peripheral organs to endotoxin-induced systemic inflammatory response in caspase-1-deficient mice

OBJECTIVES

Caspase-1 (casp1), a key protease involved in the systemic inflammatory response syndrome (SIRS), controls the brain expression of a set of eight genes: Nos2 and Ptgs2 (nitric oxide synthase 2 and prostaglandin-endoperoxide synthase 2, two inducible enzymes), Cxcl1 and Cxcl10 (C-X-C motif chemokine ligand 1 and ligand 10), Tgtp and Gbp2 (T cell-specific GTPase 1 and guanylate-binding protein 2, two GTPases), Adamts1 (a disintegrin-like and metallopeptidase with thrombospondin type 1 motif, 1, a metalloprotease) and Il1rn (interleukin-1 receptor antagonist). Our objective was to ascertain whether casp1 also controlled the peripheral expression of these genes and, if so, to compare their central versus peripheral patterns of gene expression in immune and endocrine tissues during SIRS.

METHODS

Wild-type (wt) and casp1 knockout (casp1(-/-)) mice were injected with either saline or a high dose of endotoxin/lipopolysaccharide (LPS; 800 μg/mice i.p.). Saline-injected mice were immediately euthanized after injection, whereas LPS-injected mice were sacrificed 6 and 12 h after LPS administration. Hippocampal, splenic and adrenal gene expressions were determined by real-time PCR.

RESULTS

Overall, casp1(-/-) mice showed a lower inflammatory response than wt mice. The expression levels of powerful proinflammatory factors such as Nos2 and Ptgs2 was reduced in casp1(-/-) mice. Moreover, a hierarchical clustering analysis aimed at studying patterns of gene coexpression revealed large alterations in the hippocampal pattern of casp1(-/-) mice. Surprisingly, the expression of Adamts1 was increased in the hippocampus and adrenals of casp1(-/-) mice.

CONCLUSIONS

The resilience of casp1(-/-) mice to SIRS lethality is associated with a lower inflammatory response, loss of hippocampal gene coexpression patterns, and increased hippocampal Adamts1 gene expression. The latter might be beneficial for casp1(-/-) mice, since ADAMTS1 is likely to play a role in neuronal plasticity. The mechanisms described here may help the development of either novel biomarkers or therapeutic targets against SIRS/sepsis.

Hormonal synchronization of lipopolysaccharide-induced hypothermic response in rats

BACKGROUND

Recent experimental evidence suggests that lipopolysaccharide (LPS)-induced hypothermia is an adaptive thermoregulatory strategy against immunological challenge in rats. We hypothesized that the hormones which are predominantly responsible for energy homeostasis may have efferent signaling roles for development of the hypothermia.

AIM

The aim of the study was to evaluate the changes of hypothalamic-pituitary-thyroid (HPT) and hypothalamic- pituitary-adrenal (HPA) axis hormones, leptin and erythropoietin at various phases of LPS-induced hypothermia such as the initial phase, nadir and the end of the response in blood sampled rats.

MATERIAL AND METHODS

Body temperature of adult male albino Wistar rats was recorded by biotelemetry. E. coli O111:B4 LPS (250 μg/kg, ip) was injected alone or with SC-560, a cyclooxygenase-1 selective inhibitor (1 mg/kg, sc).

RESULTS

Serum FT4 levels elevated at the initial phase, but FT3 levels decreased at nadir and remained low at the end of the response. Meanwhile, no change was observed in TSH levels. Serum adrenocorticotropic hormone (ACTH) levels reduced at the initial phase and serum corticosterone levels decreased at nadir without any change in serum corticotropin-releasing hormone (CRH) levels throughout the hypothermia. Serum leptin levels increased only at the end of the response. No change was observed in the levels of serum erythropoietin. SC-560 treatment abolished both LPS-induced hypothermia and respective hormonal changes.

CONCLUSION

Data suggest that HPT axis hormones may contribute to development of LPS-induced hypothermia in rats. Data also support the view that leptin may have a role for the recovery of hypothermic response.

C-reactive protein increases BBB permeability: implications for obesity and neuroinflammation

BACKGROUND/AIMS

Acute phase C-reactive protein (CRP), elevated in obesity and inflammation, is a major binding protein for leptin. It is thought that CRP contributes to leptin resistance by preventing leptin from crossing the blood-brain barrier (BBB). Here we determined how CRP interacts with the BBB and whether it deters leptin from reaching CNS targets.

METHODS

BBB permeability, compartmental distribution, tracer stability, and expression of tight junction protein and inflammatory marker were determined.

RESULTS

CRP was stable in blood, but did not permeate the BBB in trace amounts. However, it increased paracellular permeability at a higher dose. Agouti viable (A(vy)) mice with adult-onset obesity show higher CRP entry into the brain. CRP did not permeate hCMEC/D3 cells nor change zona occludin-1 or cyclooxygenase-2 expression. An intermediate dose of CRP had no effect on leptin transport across the BBB after co-treatment. Thus, acute interactions between CRP and leptin at the BBB level were negligible and did not explain the leptin resistance seen in obesity.

CONCLUSIONS

The interactions of CRP and the BBB are a two-phase process, with increased paracellular permeability at a high dose that enables its entry into the CNS and serves to induce reactive gliosis and impair CNS function.

Role of the blood-brain barrier in the evolution of feeding and cognition

The blood-brain barrier (BBB) regulates the blood-to-brain passage of gastrointestinal hormones, thus informing the brain about feeding and nutritional status. Disruption of this communication results in dysregulation of feeding and body weight control. Leptin, which crosses the BBB to inform the CNS about adiposity, provides an example. Impaired leptin transport, especially coupled with central resistance, results in obesity. Various substances/conditions regulate leptin BBB transport. For example, triglycerides inhibit leptin transport. This may represent an evolutionary adaptation in that hypertriglyceridemia occurs during starvation. Inhibition of leptin, an anorectic, during starvation could have survival advantages. The large number of other substances that influence feeding is explained by the complexity of feeding. This complexity includes cognitive aspects; animals in the wild are faced with cost/benefit analyses to feed in the safest, most economical way. This cognitive aspect partially explains why so many feeding substances affect neurogenesis, neuroprotection, and cognition. The relation between triglycerides and cognition may be partially mediated through triglyceride's ability to regulate the BBB transport of cognitively active gastrointestinal hormones such as leptin, insulin, and ghrelin.

Reward mechanisms in obesity: new insights and future directions

Food is consumed in order to maintain energy balance at homeostatic levels. In addition, palatable food is also consumed for its hedonic properties independent of energy status. Such reward-related consumption can result in caloric intake exceeding requirements and is considered a major culprit in the rapidly increasing rates of obesity in developed countries. Compared with homeostatic mechanisms of feeding, much less is known about how hedonic systems in brain influence food intake. Intriguingly, excessive consumption of palatable food can trigger neuroadaptive responses in brain reward circuitries similar to drugs of abuse. Furthermore, similar genetic vulnerabilities in brain reward systems can increase predisposition to drug addiction and obesity. Here, recent advances in our understanding of the brain circuitries that regulate hedonic aspects of feeding behavior will be reviewed. Also, emerging evidence suggesting that obesity and drug addiction may share common hedonic mechanisms will also be considered.

Dopamine D2 receptors in addiction-like reward dysfunction and compulsive eating in obese rats

We found that development of obesity was coupled with emergence of a progressively worsening deficit in neural reward responses. Similar changes in reward homeostasis induced by cocaine or heroin are considered to be crucial in triggering the transition from casual to compulsive drug-taking. Accordingly, we detected compulsive-like feeding behavior in obese but not lean rats, measured as palatable food consumption that was resistant to disruption by an aversive conditioned stimulus. Striatal dopamine D2 receptors (D2Rs) were downregulated in obese rats, as has been reported in humans addicted to drugs. Moreover, lentivirus-mediated knockdown of striatal D2Rs rapidly accelerated the development of addiction-like reward deficits and the onset of compulsive-like food seeking in rats with extended access to palatable high-fat food. These data demonstrate that overconsumption of palatable food triggers addiction-like neuroadaptive responses in brain reward circuits and drives the development of compulsive eating. Common hedonic mechanisms may therefore underlie obesity and drug addiction.

Dopamine D2 receptors in addiction-like reward dysfunction and compulsive eating in obese rats

We found that development of obesity was coupled with the emergence of a progressively worsening brain reward deficit. Similar changes in reward homeostasis induced by cocaine or heroin is considered a critical trigger in the transition from casual to compulsive drug-taking. Accordingly, we detected compulsive-like feeding behavior in obese but not lean rats, measured as palatable food consumption that was resistant to disruption by an aversive conditioned stimulus. Striatal dopamine D2 receptors (D2R) were downregulated in obese rats, similar to previous reports in human drug addicts. Moreover, lentivirus-mediated knockdown of striatal D2R rapidly accelerated the development of addiction-like reward deficits and the onset of compulsive-like food seeking in rats with extended access to palatable high-fat food. These data demonstrate that overconsumption of palatable food triggers addiction-like neuroadaptive responses in brain reward circuitries and drives the development of compulsive eating. Common hedonic mechanisms may therefore underlie obesity and drug addiction.

Leptin modulates cell morphology and cytokine release in microglia

The appetite suppressing hormone, leptin is now established as an important component of the immune response to pathogens partly via the induction of brain IL-1beta. We have previously demonstrated that this hormone acts on microglia to induce the release of IL-1beta through actions on its functional receptors. In the present study, we extended these findings by demonstrating that leptin's action on microglia is that of a modulator rather than a direct trigger of inflammation. Using primary microglia cultures prepared from rat brain we show that pre-incubation of these cells with leptin for 24h prior to treatment with LPS increased the IL-1beta output 2-fold. This effect was not limited to IL-1beta but was also true for another cytokine, TNF-alpha and chemokines such as CINC-1 and MIP-2. The role of leptin in potentiating the microglial response to LPS appeared to be linked to morphological changes rendering the microglia more reactive. These results suggest that leptin has an important role in microglial function in inflammation and given that its circulating levels fluctuate across a number of conditions, these findings can have important implications for an individual's ability to mount an efficient and complete response to invading pathogens.

Bromocriptine reduces augmented thyrotropin secretion in obese premenopausal women

CONTEXT

Diurnal TSH secretion is enhanced in obese premenopausal women. Dopamine inhibits TSH secretion through activation of dopamine D(2) receptors (D(2)R). Dopamine D(2)R availability in the brain is reduced in obese humans in proportion to body adiposity. We hypothesized that deficient dopamine D(2)R signaling is involved in the enhanced TSH secretion associated with obesity.

OBJECTIVE

The effect of short-term bromocriptine treatment on spontaneous TSH secretion in obese women was studied while body weight and caloric intake remained constant.

DESIGN AND SETTING

We conducted a prospective, fixed-order, crossover study in a Clinical Research Center.

PARTICIPANTS

Seventeen obese women (body mass index, 33.2 +/- 0.6 kg/m(2)) were studied twice in the early follicular phase of their menstrual cycle.

INTERVENTION

Subjects were treated for 8 d with placebo and bromocriptine.

MAIN OUTCOME MEASURE(S)

Blood was collected for 24 h at 10-min intervals, and TSH and leptin were analyzed with deconvolution and correlation techniques, approximate entropy, and cosine regression.

RESULTS

Bromocriptine reduced 24-h TSH secretion (placebo, 29.8 +/- 4.6 mU/liter . 24 h, vs. bromocriptine, 22.4 +/- 3.7 mU/liter . 24 h; P = 0.001), whereas free T(4) and total T(3) concentrations did not change. Bromocriptine administration reduced the mesor and amplitude of the 24-h rhythm without resetting the phase. The regularity of the subordinate TSH pattern and synchrony between leptin and TSH were unaffected by bromocriptine.

CONCLUSION

Activation of dopamine D(2)R by bromocriptine reverses enhanced diurnal TSH secretion in obese women. Thus, reduced dopaminergic neuronal signaling might be involved in the perturbation of the thyrotrope hormonal axis in obese premenopausal women.

Weakened [corrected] taste-LPS association during endotoxin tolerance

In naive individuals, the administration of bacterial lipopolysaccharide (LPS) provokes a rapid systemic increase in pro-inflammatory cytokines such as tumor necrosis factor (TNF)-alpha, interleukin (IL)-1beta and IL-6, inducing an acute phase response including sickness behavior. Strong associative learning occurs when relevant gustatory/olfactory stimuli precede the activation of the immune system, affecting long-term individual food selection and nutritional strategies. Repeated LPS administration results in the development of an endotoxin tolerance status, characterized by a drastic reduction in the LPS-induced cytokine response. Here we investigated how the postprandial categorization of a relevant taste (0.2% saccharin) changed after administration of a high dose of LPS (0.5 mg/kg i.p.) in LPS-tolerant animals. Determination of the consummatory fluid intake revealed that, in contrast to LPS-naive rats, taste-LPS association did not occur during endotoxin tolerance. Ninety minutes after the single association trial, the plasma responses of TNF-alpha, IL-1beta and IL-6 were completely blunted in LPS-tolerant animals, which also resulted in low LPS-adipsogenic and LPS-anorexic effects. These findings indicate that an identical immune challenge can result in completely different neuro-behavioral consequences depending on the immune history of the individual, thus revealing part of the complex interconnection between the immune and neuro-endocrine systems in regulating food selection and consumption during the infectious process.

Neonatal immune challenge does not affect body weight regulation in rats

The perinatal environment plays a crucial role in programming many aspects of adult physiology. Myriad stressors during pregnancy, from maternal immune challenge to nutritional deficiency, can alter long-term body weight set points of the offspring. In light of the increasing concern over body weight issues, such as obesity and anorexia, in modern societies and accumulating evidence that developmental stressors have long-lasting effects on other aspects of physiology (e.g., fever, pain), we explored the role of immune system activation during neonatal development and its impact on body weight regulation in adulthood. Here we present a thorough evaluation of the effects of immune system activation (LPS, 100 microg/kg ip) at postnatal days 3, 7, or 14 on long-term body weight, adiposity, and body weight regulation after a further LPS injection (50 microg/kg ip) or fasting and basal and LPS-induced circulating levels of the appetite-regulating proinflammatory cytokine leptin. We show that neonatal exposure to LPS at various times during the neonatal period has no long-term effects on growth, body weight, or adiposity. We also observed no effects on body weight regulation in response to a short fasting period or a further exposure to LPS. Despite reductions in circulating leptin levels in response to LPS during the neonatal period, no long-term effects on leptin were seen. These results convincingly demonstrate that adult body weight and weight regulation are, unlike many other aspects of adult physiology, resistant to programming by a febrile-dose neonatal immune challenge.

Caspase 1 deficiency reduces inflammation-induced brain transcription

The systemic inflammatory response syndrome (SIRS) is a life-threatening medical condition characterized by a severe and generalized inflammatory state that can lead to multiple organ failure and shock. The CNS regulates many features of SIRS such as fever, cardiovascular, and neuroendocrine responses. Central and systemic manifestations of SIRS can be induced by LPS or IL-1beta administration. The crucial role of IL-1beta in inflammation has been further highlighted by studies of mice lacking caspase 1 (casp1, also known as IL-1beta convertase), a protease that cleaves pro-IL-1beta into mature IL-1beta. Indeed, casp1 knockout (casp1(-/-)) mice survive lethal doses of LPS. The key role of IL-1beta in sickness behavior and its de novo expression in the CNS during inflammation led us to test the hypothesis that IL-1beta plays a major role modulating the brain transcriptome during SIRS. We show a gene-environment effect caused by LPS administration in casp1(-/-) mice. During SIRS, the expression of several genes, such as chemokines, GTPases, the metalloprotease ADAMTS1, IL-1RA, the inducible nitric oxide synthase, and cyclooxygenase-2, was differentially increased in casp1(-/-) mice. Our findings may contribute to the understanding of the molecular changes that take place within the CNS during sepsis and SIRS and the development of new therapies for these serious conditions. Our results indicate that those genes may also play a role in several neuropsychiatric conditions in which inflammation has been implicated and indicate that casp1 might be a potential therapeutic target for such disorders.

The role of the vagus nerve in mediating the long-term anorectic effects of leptin

Leptin, the product of the obese (ob) gene, is mainly known for its regulatory role of energy balance by direct activation of hypothalamic receptors. Recently, its function in the acute control of food intake was additionally attributed to activation of the vagus nerve to regulate meal termination. Whether vagal afferent neurones are involved in longer term effects of leptin on food intake, however, remains undetermined. Using vagotomised (VGX) rats, we sought to clarify the contributions of vagal afferents in mediating the long-lasting effect of leptin on appetite suppression. Intraperitoneal (i.p.) injection of leptin (3.5 mg/kg) attenuated food intake at 4, 6, 8 and 24 h and body weight at 24 h postinjection in SHAM-operated rats; however, this response was not abrogated by vagotomy. In a separate study using immunohistochemistry, we observed leptin-induced Fos expression in the nucleus tractus solitarii, a brain structure where vagal afferent fibres terminate. This signal was not attenuated in VGX animals compared to the SHAM group. Moreover, leptin treatment led to a similar level of nuclear STAT3 translocation, a marker of leptin signalling, in the hypothalami of SHAM and VGX animals. In addition to the effects of leptin, vagotomy surgery itself resulted in a decrease of 24 h food intake. Analyses of brains from saline-treated VGX animals revealed a significant induction of Fos in the nucleus tractus solitarii and changes in agouti-related peptide and pro-opiomelanocortin mRNA expression in the hypothalamus compared to their SHAM counterparts, indicating that the vagotomy surgery itself induced a modification of brain activity in areas involved in regulating appetite. Collectively, our data suggest that vagal afferents do not constitute a major route of mediating the regulatory effect of leptin on food intake over a period of several hours.

From Neuroendocrinology to Neuroimmunomodulation - a tribute to Prof. Dr. Samuel McCann

One of the leading experts in the field of Neuroendocrinology and Neuroimmunmodulation, Samuel Mac Donald McCann, known by all his friends as 'Don', passed away in 2007. This article pays tribute to his outstanding scientific contribution and a glimpse on his fascinating personality. A member of the National Academy of Sciences of the United States and pioneer in the field of neuroendocrine regulation, he identified numerous hormones and peptides and set the stage for basic concepts in physiology and clinical medicine.

Leptin: at the crossroads of energy balance and systemic inflammation

In addition to playing a central role in energy homeostasis, leptin is also an important player in the inflammatory response. Systemic inflammation is accompanied by fever (less severe cases) or hypothermia (more severe cases). In leptin-irresponsive mutants, the hypothermia of systemic inflammation is exaggerated, presumably due to the enhanced production and cryogenic action of tumor necrosis factor (TNF)-alpha. Mechanisms that exaggerate hypothermia can also attenuate fever, particularly in a cool environment. Another common manifestation of systemic inflammation is behavioral depression. Along with the production of interleukin (IL)-1beta, this manifestation is exaggerated in leptin-irresponsive mutants. The enhanced production of TNF-alpha and IL-1beta may be due, at least in part, to insufficient activation of the anti-inflammatory hypothalamo-pituitary-adrenal axis by immune stimuli in the absence of leptin signaling. In experimental animals and humans that are responsive to leptin, suppression of leptin production under conditions of negative energy balance (e.g., fasting) can exaggerate both hypothermia and behavioral depression. Since these manifestations aid energy conservation, exaggeration of these manifestations under conditions of negative energy balance is likely to be beneficial.

Leptin induces cyclooxygenase-2 via an interaction with interleukin-1beta in the rat brain

In addition to its central effects on appetite regulation, leptin has been implicated in immune function and inflammation. Previous data suggested that leptin acts as an inflammatory signal within the brain, as exogenously administered leptin induced fever, a typical brain-regulated inflammatory response. The present study aimed to delineate the inflammatory actions and cellular targets of leptin in the brain by examining its effects on the expression of interleukin (IL)-1beta and cyclooxygenase (COX)-2, two important inflammatory components of the fever response. Intracerebroventricular injection of leptin (5 microg/rat) induced IL-1beta and COX-2 mRNA and protein in the hypothalamus between 1 and 3 h after treatment as determined by reverse transcription-polymerase chain reaction and immunohistochemistry. Coinjection of IL-1 receptor antagonist (100 microg/rat, intracerebroventricular) attenuated leptin-induced COX-2, whereas IL-1 receptor antagonist had no effect on endogenous IL-1beta levels, suggesting that leptin induces COX-2 via, at least partly, IL-1beta action. IL-1beta protein expression was induced in macrophages in the meningis and perivascular space after leptin treatment, whereas COX-2 induction was observed in endothelial cells, indicating the roles for these non-neuronal cells in mediating inflammatory actions of leptin. In addition, neutralization of endogenous circulating leptin with anti-leptin antiserum attenuated intraperitoneal lipopolysaccharide (100 microg/kg)-induced brain IL-1beta and COX-2 upregulation, suggesting that leptin indeed acts as an inflammatory signal to the brain during systemic inflammation. These findings are in contrast to the effects of leptin on appetite regulation where it is believed to act primarily on neurons, thus presenting a distinct anatomical basis for the inflammatory and appetite regulatory actions of leptin in the brain.

Nitric oxide-induced downregulation of leptin production by 3T3-L1 adipocytes

Leptin secreted mainly by adipocytes plays an important role in insulin sensitivity in metabolic syndrome. Inducible nitric oxide synthase (iNOS) in 3T3-L1 adipocytes is induced by lipopolysaccharide (LPS) and several proinflammatory cytokines such as tumor necrosis factor-alpha and interferon-gamma (IFN-gamma). Because the role of iNOS-derived nitric oxide (NO) in adipocyte function has not been fully clarified, the question that we addressed in the present study was whether iNOS-derived NO is involved in regulation of leptin secretion by adipocytes. Incubation of 3T3-L1 adipocytes for 12h with a mixture of IFN-gamma and LPS caused not only a 55% reduction in leptin secretion and a 52% reduction in leptin mRNA, but also significant induction of iNOS at both protein and mRNA levels. Inhibition of leptin secretion that had been induced by the IFN-gamma-LPS mixture was completely nullified by NOS inhibitors such as Nomega-monomethyl-L-arginine and aminoguanidine. Treatment of adipocytes with NO donors such as an NONOate and S-nitrosoglutathione produced an effect on leptin secretion similar to that of the IFN-gamma-LPS mixture. It is likely therefore that NO mediates downregulation of leptin caused by the IFN-gamma-LPS mixture in 3T3-L1 adipocytes, which suggests an important role for NO in adipocyte functions.

Activation of dopamine D2 receptors lowers circadian leptin concentrations in obese women

CONTEXT

Leptin release is regulated by factors other than fat mass alone. Previous observations provide indirect evidence for an inhibitory effect of dopaminergic neurotransmission on leptin secretion. This study was done to establish the effect of bromocriptine treatment on circadian plasma leptin concentrations in obese humans.

OBJECTIVE

The objective of the study was to study the acute effects of bromocriptine (a D2R agonist) on circadian leptin levels in obese women, whereas body weight and caloric intake remained constant.

DESIGN

This was a prospective, single-blind, crossover study (2004).

SETTING

The study was conducted at a clinical research center.

PARTICIPANTS

Eighteen healthy obese women (body mass index 33.2 +/- 0.6 kg/m(2)) were studied twice in the early follicular phase of their menstrual cycle.

INTERVENTION(S)

Treatment consisted of bromocriptine or placebo for 8 d.

MAIN OUTCOME MEASURE(S)

Blood was collected during 24 h at 20-min intervals for determination of leptin concentrations at the last day of medical treatment (bromocriptine or placebo). Mean 24-h serum concentrations were determined for insulin, glucose, free fatty acids, and triglycerides.

RESULTS

Short-term treatment with bromocriptine reduced leptin concentration (placebo 33.6 +/- 2.5 vs. bromocriptine 30.5 +/- 2.5 ng/liter, P = 0.03). Free fatty acid concentrations were increased by treatment with bromocriptine. The increase of free fatty acids was inversely related with the decline of leptin levels. The decline of glucose, insulin, or prolactin concentrations in response to bromocriptine was not correlated with the reduction of leptin.

CONCLUSION

Activation of dopamine D2 receptors by bromocriptine lowers circulating leptin levels in obese women, which suggests that dopaminergic neurotransmission is involved in the control of leptin release in humans.

Activation of the hypothalamic-pituitary-adrenal axis and autonomic nervous system during inflammation and altered programming of the neuroendocrine-immune axis during fetal and neonatal development: lessons learned from the model inflammagen, lipopolysaccharide

The hypothalamic-pituitary-adrenal axis (HPAA) and autonomic nervous system (ANS) are both activated during inflammation as an elaborate multi-directional communication pathway designed to restore homeostasis, in part, by regulating the inflammatory and subsequent immune response. During fetal and neonatal development programming of the HPAA, ANS and possibly the immune system is influenced by signals from the surrounding environment, as part of an adaptive mechanism to enhance the survival of the offspring. It is currently hypothesized that if this programming is either misguided, or the individual's environment is drastically altered such that neuroendocrine programming becomes maladaptive, it may contribute to the pathogenesis of certain diseases. Current research, suggests that exposure to inflammatory signals during critical windows of early life development may influence the programming of various genes within the neuroendocrine-immune axis. This review will provide, (1) an overview of the HPAA and ANS pathways that are activated during inflammation, highlighting studies that have used lipopolysaccharide as a model inflammagen and, (2) evidence to support the hypothesis that inflammatory stress during fetal and neonatal development can alter programming of the neuroendocrine-immune axis, influencing stress and immune responsiveness, and possibly disease resistance later in life.

Effect of leptin on liver alcohol dehydrogenase

The effect of leptin on liver alcohol dehydrogenase (ADH) was determined in male rats. Administration of one or three daily doses of leptin (1microg/g of body weight intraperitoneally) increased ADH activity. Leptin enhanced ADH synthesis without an effect on ADH degradation. Leptin did not change ADH mRNA, indicating that the effect of leptin in enhancing ADH occurs at the post-transcriptional level. Leptin increased eukaryotic initiation factor (eIF) 2alpha, eIF2B activity, and the eIF4E-eIF4G complex, while it decreased the inhibitory complex of eIF4E with the eIF4E-binding protein-1 (4E-BP1). Leptin increased mammalian target of rapamycin (mTor) that phosphorylates 4E-BP1. In conclusion, leptin increases liver ADH activity and ADH protein due to an increase in synthesis which occurs at the post-transcriptional level. The effect of leptin in enhancing translational initiating factors may be of significance in the regulation not only of ADH but also of many other proteins.

Induction of galanin-like peptide gene expression in the arcuate nucleus of the rat after acute but not chronic inflammatory stress

Galanin-like peptide (GALP) has been recently isolated from the porcine hypothalamus. The GALP mRNA is restricted to neurons in the hypothalamic arcuate nucleus (Arc) and pituicytes in the posterior pituitary gland (PP), but physiological functions of the GALP remains unclear in both areas. We examined the effects of acute and chronic inflammatory stresses on the GALP mRNA levels in the rat Arc using in situ hybridization histochemistry. Intraperitoneal (i.p.) injection of bacterial endotoxin lipopolysaccharide (LPS) caused a marked increase of the GALP mRNA levels in the Arc. The effects of i.p. injection of LPS on the GALP mRNA levels in the Arc were significantly attenuated by pretreatment with i.p. injection of indomethacin cyclooxygenase inhibitor. Adjuvant arthritis caused by subcutaneous (s.c.) injection of heat-killed Mycobacterium butyricum as chronic inflammatory stress did not affect the GALP mRNA levels in the Arc, though the GALP mRNA levels in the pituicytes of the PP were markedly increased by two peaks at 12 h and 15 days after s.c. injection of heat-killed M. butyricum. Enzymeimmunoassay showed that the plasma concentration of GALP was not affected by these inflammatory stresses. These results suggest that acute inflammatory stress might be a potent stimulant to increase the GALP mRNA levels in the Arc of the rat via synthesis of prostaglandins.

Lipopolysaccharide-induced leptin synthesis and release are differentially controlled by alpha-melanocyte-stimulating hormone

OBJECTIVE

Since alpha-melanocyte-stimulating hormone (alpha-MSH) inhibits the synthesis and release of proinflammatory cytokines and stimulates the synthesis and release of anti-inflammatory cytokines, and leptin is a cytokine that has anti-inflammatory actions in the presence of lipopolysaccharide (LPS), we hypothesized that alpha-MSH increases leptin synthesis and release.

METHODS

alpha-MSH or 0.9% NaCl (saline) were injected intraperitoneally 15 min prior to intravenous injection of 0.5 ml of saline or LPS (0.15 mg/kg). Thereafter, repeated blood samples were withdrawn over a period of 6 h and plasma leptin concentrations determined. The rats were sacrificed at 6 h and leptin mRNA was measured in epididymal fat pads.

RESULTS

Plasma leptin concentrations of the saline-injected control group were unaltered during the 6 h, whereas in the LPS group, leptin was unaltered between 0 and 30 min and thereafter progressively increased between 30 and 360 min by 2.5-fold. alpha-MSH slightly increased plasma leptin concentrations by 15 min and then increased them further by 120 min, after which they declined towards baseline. The pattern of plasma leptin concentrations in the alpha-MSH + LPS group was similar to that of the LPS group, except that higher concentrations were observed at 120 min in the rats injected with alpha-MSH + LPS. LPS increased leptin mRNA by 3-fold at 6 h, whereas it was unaffected in the MSH-treated animals. On the contrary, alpha-MSH completely blocked the LPS-induced leptin mRNA.

CONCLUSIONS

Our results suggest that alpha-MSH increased leptin release without altering its synthesis, but when LPS increased release and synthesis of leptin, alpha-MSH, although further increasing release, blocked the enhanced synthesis of leptin elicited by LPS.

Leptin fluctuates in intestinal ischemia-reperfusion injury as inflammatory cytokine

As leptin is an active mediator mainly secreted by adipose tissue and is closely related with energy metabolism, we evaluate both the changes of leptin levels in serum and adipose tissue with a concise radioimmunoassay and the changes of leptin mRNA expression in adipose tissue with RT-PCR, during the severe metabolic impediment in rat intestinal ischemia-reperfusion (I/R) injury. Results show that not only leptin levels in serum and adipose tissue but also its mRNA expression in adipose tissue undergo a fluctuation according to different injury times. Therefore, we conclude that leptin has a time-dependent response to acute inflammatory stimuli and acts as an anti-inflammatory cytokine.

Reduced adipose tissue mass and hypoleptinemia in iNOS deficient mice: effect of LPS on plasma leptin and adiponectin concentrations

The aim of this study was to evaluate the impact of the lack of inducible NO synthase (iNOS) on body weight and adipose tissue mass as well as on plasma leptin and adiponectin in basal conditions and 6 h after lipopolysaccharide (LPS) administration in mice. Body weight was not different among male, six-week-old wild-type (WT) and iNOS-/- animals. However, the amount of epididymal white adipose tissue (EWAT) in iNOS-/- mice was significantly reduced (P<0.05). Circulating leptin and leptin mRNA in EWAT were decreased in iNOS-/- mice (P<0.05 and P<0.01, respectively). Plasma adiponectin and adiponectin mRNA were unchanged. LPS administration increased plasma leptin in both genotypes (P<0.05). Neither genotype nor treatment changed plasma adiponectin. In summary, iNOS-/- mice exhibited normal body weight but reduced adipose mass accompanied by hypoleptinemia. Leptin responsiveness to LPS in iNOS-/- mutants is preserved, showing that the LPS-induced rise in leptin is independent of the presence of functional iNOS. In addition, iNOS deficiency or LPS does not influence expression and circulating levels of adiponectin.

Circulating leptin mediates lipopolysaccharide-induced anorexia and fever in rats

Anorexia and fever are important features of the host's response to inflammation that can be triggered by the bacterial endotoxin lipopolysaccharide (LPS) and the appetite suppressant leptin. Previous studies have demonstrated that LPS induces leptin synthesis and secretion in the periphery, and that the action of leptin on appetite suppression and fever are dependent on brain interleukin (IL)-1beta. However, the role of leptin as a neuroimmune mediator of LPS-induced inflammation has not been fully elucidated. To address this issue, we neutralized circulating leptin using a leptin antiserum (LAS) and determined how this neutralization affected LPS-induced anorexia, fever and hypothalamic IL-1beta. Adult male rats were separated into four treatment groups, namely LPS + normal sheep serum (NSS), LPS + LAS, saline + LAS and saline + NSS. Intraperitoneal injection of LPS (100 microg kg(-1)) induced a significant reduction in food intake and body weight, which were significantly reversed in the presence of LAS (1 ml kg(-1)), 8 and 24 h after treatment. In addition, LPS-induced fever was significantly attenuated by LAS over the duration of the fever response (8 h). Lipopolysaccharide induced an increase of circulating IL-6, another potential circulating pyrogen, which was not affected by neutralization of leptin at 2 h. Interleukin-1beta mRNA at 1 and 8 h, and IL-1 receptor antagonist (ra) at 2 h were significantly upregulated in the hypothalamus of LPS-treated animals. The induction of these cytokines was attenuated in the presence of LAS. These results are the first to demonstrate that leptin is a circulating mediator of LPS-induced anorexia and fever, probably through a hypothalamic IL-1beta-dependent mechanism.

Effects of lipopolysaccharide on leptin transport across the blood–brain barrier

Leptin is a 16-kDa protein secreted by fat cells and transported into the brain where it decreases appetite and increases body temperature. Leptin transport is saturable and regulated by epinephrine, triglycerides, and starvation. Lipopolysaccharide (LPS) is derived from bacterial cell walls and also decreases appetite and increases body temperature. LPS is known to increase leptin levels in serum and to affect the passage of other regulatory proteins across the blood-brain barrier (BBB). Here, we examined the ability of LPS, at doses which induce weight loss, to modify the BBB transport of radioactive leptin (I-Lep). The transport rate of intravenously injected I-Lep was decreased by 50-60% from 8 to 12 h after a single i.p. injection of LPS (3 mg/kg). The effect of LPS was dose-dependent. In comparison to the brain/serum ratio, the baseline cerebrospinal fluid (CSF)/serum ratio for I-Lep was much lower and not inhibited by LPS. LPS did not affect I-Lep transport when studied by the brain perfusion method nor was Ob-Ra mRNA expression in isolated brain microvessels altered, demonstrating that a circulating factor rather than altered BBB function was responsible for inhibition. Brain perfusion showed that LPS was not this factor. Serum leptin was doubled and serum triglycerides increased by 44% after LPS administration, suggesting these to be the circulating inhibitory factors. In conclusion, a single dose of LPS has long-lasting effects on the transport of serum leptin across the BBB that are likely mediated through self-inhibition and triglycerides.

Hormone and metabolic factors associated with leptin mRNA expression in pre- and postmenopausal women

Recent information has extended leptin's action, beyond the control of appetite, to various sites of metabolic regulation. To better understand leptin's role we studied its production in subcutaneous and visceral fat compartments before and after menopause. During elective abdominal surgery, biopsies of subcutaneous and omental tissues were taken from 20 women at pre- (BMI 28.4 +/- 4.5 kg/m2) and 10 at postmenopause (BMI 30.6 +/- 7.7 kg/m2). In both groups serum leptin levels were similar, and highly correlated with BMI. In subcutaneous adipose tissue, leptin mRNA expression was significantly higher in pre- than in postmenopausal women (50.4 +/- 20.5 amol/microg total RNA versus 34.5 +/- 24.9 amol/microg total RNA, respectively). Leptin mRNA expression in subcutaneous tissue was independently correlated with fasting glucose (R = 0.89, P < 0.006) at premenopause, and with serum estradiol (R = 0.77, P < 0.04) at postmenopause. Leptin mRNA expression in visceral fat was correlated with DHEAS (R = 0.86, P < 0.001), at premenopause. These results indicate that in both compartments, leptin production is sensitive to different but overlapping stimuli, conveying information about energy availability to central and peripheral sites under different conditions of estrogen exposure.

Involvement of Capsaicin-Sensitive Afferent Nerves and Cholecystokinin 2/Gastrin Receptors in Gastroprotection and Adaptation of Gastric Mucosa toHelicobacter pylori-Lipopolysaccharide

Lipopolysaccharide (LPS) is one of the virulence factors in the Helicobacter pylori (Hp)-infected stomach, but it remains unknown whether single and prolonged pretreatment with Hp-LPS can affect the course of gastric damage induced by aspirin (ASA). We compared the effects of Hp-LPS with those induced by LPSs isolated from intestinal Bacteroides fragilis, Yersinia enterocolitica, and Campylobacter jejuni applied for 4 days on acute ASA-induced gastric lesions in rats. The area of ASA-induced gastric lesions, gastric blood flow (GBF), expression of mRNA and protein of leptin and plasma leptin, gastrin, interleukin-1beta, and tumor necrosis factor-alpha levels were examined. Single (once) or repeated (five times) i.p. injections of Hp-LPS (1 mg/kg) or intestinal LPSs failed to produce macroscopic gastric damage and did not affect the GBF when compared with vehicle. Hp-LPS injected repeatedly suppressed the gastric acid secretion, up-regulated leptin mRNA and protein, and increased plasma leptin and gastrin levels. Hp-LPS significantly reduced the ASA-induced gastric damage and the accompanying decline in the GBF, and these effects were significantly attenuated by capsaicin denervation and selective antagonism of cholecystokinin-B (CCK2) receptors by RPR-102681 [N-(metoxy-3 phenyl) N-(N-methyl N-phenyl-carbamylmethyl) carbamoylmethyl]-3 ureido]-3 phenyl]-2 propronique] but not by loxiglumide, an antagonist of CCK1 receptors. We conclude that 1) daily application of Hp-LPS enhances gastric mucosal resistance against ASA damage due to the increase of GBF and the expression and release of leptin and gastrin exerting trophic and gastroprotective effects, and 2) this enhanced resistance to ASA damage in Hp-LPS-adapted stomach is mediated by the sensory afferents and specific CCK2/gastrin receptors.

Endotoxin stimulates leptin in the human and nonhuman primate

Leptin, which plays a key role in regulating energy homeostasis, may also modulate the inflammatory response. An inflammatory challenge with endotoxin has been shown to stimulate leptin release in the rodent. This finding has not been reproduced in humans or in nonhuman primates, although leptin levels have been reported to increase in septic patients. We have therefore examined the effects of endotoxin injection on plasma leptin levels in nine ovariectomized monkeys and four postmenopausal women. In an initial study in five monkeys, mean leptin levels did not increase during the first 5 h after endotoxin treatment, but did increase significantly from 6.4 +/- 2.1 ng/ml at baseline to 12.3 +/- 4.4 ng/ml at 24 h (P = 0.043). In a second study, a significant increase in leptin over time was noted after endotoxin treatment (P < 0.001); leptin release during the 16- to 24-h period after endotoxin injection was 48% higher than during the control period (P = 0.043). A similar stimulatory effect of endotoxin on leptin was observed when monkeys received estradiol replacement. In a third study, repeated injections of endotoxin over a 3-d period stimulated IL-6, ACTH, cortisol, and leptin release (P < 0.001). Leptin increased during the first day of treatment in all animals, but only monkeys with baseline plasma leptin levels greater than 10 ng/ml exhibited a sustained increase in leptin throughout the 3-d period. There was a significant correlation (r = 0.81; P = 0.008) between the mean baseline leptin level and the percent increase in leptin over baseline on the last day of treatment. In the human subjects, plasma leptin concentrations did not change significantly during the 7-h period after endotoxin injection. However, leptin increased in all four women from a mean baseline of 8.34 +/- 3.1 to 13.1 +/- 4.3 ng/ml 24 h after endotoxin (P = 0.038). In summary, endotoxin stimulates the release of leptin into peripheral blood in the human and nonhuman primate, but the time course is different from that reported in the rodent. These results are consistent with previous reports of increased blood leptin levels in patients with sepsis. The significance of these findings and the potential role of leptin in modulating the response to inflammation in the human require further study.

Resting and circadian release of nitric oxide is controlled by leptin in male rats

Because leptin stimulates nitric oxide (NO) release from the hypothalamus and anterior pituitary gland, we hypothesized that it also might release NO from adipocytes, the principal source of leptin. Consequently, plasma concentrations of leptin and NO, estimated from its metabolites NO(3) and NO(2) (NO(3)-NO(2)), were measured in adult male rats. There was a linear increase of both leptin and NO(3)-NO(2) with body weight that was associated with a parallel rise in fat mass. These findings indicate that release of leptin and NO is directly related to adipocyte mass. Furthermore, there was a parallelism in circadian rhythm of both substances, with peaks at 0130 h and nadirs at 0730 h. Measurement of both leptin and NO(3)-NO(2) in plasma from individual rats revealed that NO(3)-NO(2) increased linearly with leptin. Incubation of epididymal fat pads with leptin or its i.v. injection in conscious rats increased NO(3)-NO(2) release. The release of NO(3)-NO(2) in vivo and in vitro exceeded that of leptin by many fold, indicating that leptin activates NO synthase. Leptin increased tumor necrosis factor (TNF)-alpha release at a 100-fold lower dose than required for NO release in vitro and in vivo, suggesting that it also may participate in leptin-induced NO release. However, because many molecules of leptin were required to release a molecule of TNF-alpha in vivo and in vitro, we believe that leptin-induced TNF-alpha release is an associated phenomenon not involved in NO production. The results support the hypothesis that adipocytes play a major role in NO release by activating NO synthase in the adipocytes and the adjacent capillary endothelium.