Vagal and splanchnic afferents are not necessary for the anorexia produced by peripheral IL-1beta, LPS, and MDP

The Microbiota and the Gut-Brain Axis in Controlling Food Intake and Energy Homeostasis

Obesity currently represents a major societal and health challenge worldwide. Its prevalence has reached epidemic proportions and trends continue to rise, reflecting the need for more effective preventive measures. Hypothalamic circuits that control energy homeostasis in response to food intake are interesting targets for body-weight management, for example, through interventions that reinforce the gut-to-brain nutrient signalling, whose malfunction contributes to obesity. Gut microbiota-diet interactions might interfere in nutrient sensing and signalling from the gut to the brain, where the information is processed to control energy homeostasis. This gut microbiota-brain crosstalk is mediated by metabolites, mainly short chain fatty acids, secondary bile acids or amino acids-derived metabolites and subcellular bacterial components. These activate gut-endocrine and/or neural-mediated pathways or pass to systemic circulation and then reach the brain. Feeding time and dietary composition are the main drivers of the gut microbiota structure and function. Therefore, aberrant feeding patterns or unhealthy diets might alter gut microbiota-diet interactions and modify nutrient availability and/or microbial ligands transmitting information from the gut to the brain in response to food intake, thus impairing energy homeostasis. Herein, we update the scientific evidence supporting that gut microbiota is a source of novel dietary and non-dietary biological products that may beneficially regulate gut-to-brain communication and, thus, improve metabolic health. Additionally, we evaluate how the feeding time and dietary composition modulate the gut microbiota and, thereby, the intraluminal availability of these biological products with potential effects on energy homeostasis. The review also identifies knowledge gaps and the advances required to clinically apply microbiome-based strategies to improve the gut-brain axis function and, thus, combat obesity.

TLR4 Signaling Selectively and Directly Promotes CGRP Release from Vagal Afferents in the Mouse

There has been a long-standing debate regarding the role of peripheral afferents in mediating rapid-onset anorexia among other responses elicited by peripheral inflammatory insults. Thus, the current study assessed the sufficiency of peripheral afferents expressing toll-like receptor 4 (TLR4) to the initiation of the anorexia caused by peripheral bacterial lipopolysaccharide (LPS). We generated a Tlr4 null (Tlr4^LoxTB) mouse in which Tlr4 expression is globally disrupted by a loxP-flanked transcription blocking (TB) cassette. This novel mouse model allowed us to restore the endogenous TLR4 expression in specific cell types. Using Zp3-Cre and Na_v1.8-Cre mice, we produced mice that express TLR4 in all cells (Tlr4^LoxTB X Zp3-Cre) and in peripheral afferents (Tlr4^LoxTB X Na_v1.8-Cre), respectively. We validated the Tlr4^LoxTB mice, which were phenotypically identical to previously reported global TLR4 knock-out mice. Contrary to our expectations, the administration of LPS did not cause rapid-onset anorexia in mice with Na_v1.8-restricted TLR4. The later result prompted us to identify Tlr4-expressing vagal afferents using in situ hybridization (ISH). In vivo, we found that Tlr4 mRNA was primarily enriched in vagal Na_v1.8 afferents located in the jugular ganglion that co-expressed calcitonin gene-related peptide (CGRP). In vitro, the application of LPS to cultured Na_v1.8-restricted TLR4 afferents was sufficient to stimulate the release of CGRP. In summary, we demonstrated using a new mouse model that vagally-expressed TLR4 is selectively involved in stimulating the release of CGRP but not in causing anorexia.

Review: Pro-inflammatory cytokines and hypothalamic inflammation: implications for insufficient feed intake of transition dairy cows

Improvements in feed intake of dairy cows entering the early lactation period potentially decrease the risk of metabolic disorders, but before developing approaches targeting the intake level, mechanisms controlling and dysregulating energy balance and feed intake need to be understood. This review focuses on different inflammatory pathways interfering with the neuroendocrine system regulating feed intake of periparturient dairy cows. Subacute inflammation in various peripheral organs often occurs shortly before or after calving and is associated with increased pro-inflammatory cytokine levels. These cytokines are released into the circulation and sensed by neurons located in the hypothalamus, the key brain region regulating energy balance, to signal reduction in feed intake. Besides these peripheral humoral signals, glia cells in the brain may produce pro-inflammatory cytokines independent of peripheral inflammation. Preliminary results show intensive microglia activation in early lactation, suggesting their involvement in hypothalamic inflammation and the control of feed intake of dairy cows. On the other hand, pro-inflammatory cytokine-induced activation of the vagus nerve transmits signalling to the brain, but this pathway seems not exclusively necessary to signal feed intake reduction. Yet, less studied in dairy cows so far, the endocannabinoid system links inflammation and the hypothalamic control of feed intake. Distinct endocannabinoids exert anti-inflammatory action but also stimulate the posttranslational cleavage of neuronal proopiomelanocortin towards β-endorphin, an orexigen promoting feed intake. Plasma endocannabinoid concentrations and hypothalamic β-endorphin levels increase from late pregnancy to early lactation, but less is known about the regulation of the hypothalamic endocannabinoid system during the periparturient period of dairy cows. Dietary fatty acids may modulate the formation of endocannabinoids, which opens new avenues to improve metabolic health and immune status of dairy cows.

Vagal Interoceptive Modulation of Motivated Behavior

In addition to regulating the ingestion and digestion of food, sensory feedback from gut to brain modifies emotional state and motivated behavior by subconsciously shaping cognitive and affective responses to events that bias behavioral choice. This focused review highlights evidence that gut-derived signals impact motivated behavior by engaging vagal afferents and central neural circuits that generally serve to limit or terminate goal-directed approach behaviors, and to initiate or maintain behavioral avoidance.

Anorexia-cachexia syndrome in hepatoma tumour-bearing rats requires the area postrema but not vagal afferents and is paralleled by increased MIC-1/GDF15

BACKGROUND

The cancer-anorexia-cachexia syndrome (CACS) negatively affects survival and therapy success in cancer patients. Inflammatory mediators and tumour-derived factors are thought to play an important role in the aetiology of CACS. However, the central and peripheral mechanisms contributing to CACS are insufficiently understood. The area postrema (AP) and the nucleus tractus solitarii are two important brainstem centres for the control of eating during acute sickness conditions. Recently, the tumour-derived macrophage inhibitory cytokine-1 (MIC-1) emerged as a possible mediator of cancer anorexia because lesions of these brainstem areas attenuated the anorectic effect of exogenous MIC-1 in mice.

METHODS

Using a rat hepatoma tumour model, we examined the roles of the AP and of vagal afferents in the mediation of CACS. Specifically, we investigated whether a lesion of the AP (APX) or subdiaphragmatic vagal deafferentation (SDA) attenuate anorexia, body weight, muscle, and fat loss. Moreover, we analysed MIC-1 levels in this tumour model and their correlation with tumour size and the severity of the anorectic response.

RESULTS

In tumour-bearing sham-operated animals mean daily food intake significantly decreased. The anorectic response was paralleled by a significant loss of body weight and muscle mass. APX rats were protected against anorexia, body weight loss, and muscle atrophy after tumour induction. In contrast, subdiaphragmatic vagal deafferentation did not attenuate cancer-induced anorexia or body weight loss. Tumour-bearing rats had substantially increased MIC-1 levels, which positively correlated with tumour size and cancer progression and negatively correlated with food intake.

CONCLUSIONS

These findings demonstrate the importance of the AP in the mediation of cancer-dependent anorexia and body weight loss and support a pathological role of MIC-1 as a tumour-derived factor mediating CACS, possibly via an AP-dependent action.

Consumption of Substances of Abuse during Pregnancy Increases Consumption in Offspring: Possible Underlying Mechanisms

Correlative human observational studies on substances of abuse have been highly dependent on the use of rodent models to determine the neuronal and molecular mechanisms that control behavioral outcomes. This is particularly true for gestational exposure to non-illicit substances of abuse, such as excessive dietary fat, ethanol, and nicotine, which are commonly consumed in our society. Exposure to these substances during the prenatal period has been shown in offspring to increase their intake of these substances, induce other behavioral changes, and affect neurochemical systems in several brain areas that are known to control behavior. More importantly, emerging studies are linking the function of the immune system to these neurochemicals and ingestion of these abused substances. This review article will summarize the prenatal rodent models used to study developmental changes in offspring caused by prenatal exposure to dietary fat, ethanol, or nicotine. We will discuss the various techniques used for the administration of these substances into rodents and summarize the published outcomes induced by prenatal exposure to these substances. Finally, this review will cover some of the recent evidence for the role of immune factors in causing these behavioral and neuronal changes.

Chronic exposure to low dose bacterial lipopolysaccharide inhibits leptin signaling in vagal afferent neurons

Bacterially derived factors are implicated in the causation and persistence of obesity. Ingestion of a high fat diet in rodents and obesity in human subjects is associated with chronic elevation of low plasma levels of lipopolysaccharide (LPS), a breakdown product of Gram-negative bacteria. The terminals of vagal afferent neurons are positioned within the gut mucosa to convey information from the gut to the brain to regulate food intake and are responsive to LPS. We hypothesized that chronic elevation of LPS could alter vagal afferent signaling. We surgically implanted osmotic mini-pumps that delivered a constant, low-dose of LPS into the intraperitoneal cavity of rats (12.5 μg/kg/hr for 6 weeks). LPS-treated rats developed hyperphagia and showed marked changes in vagal afferent neuron function. Chronic LPS treatment reduced vagal afferent leptin signaling, characterized by a decrease in leptin-induced STAT3 phosphorylation. In addition, LPS treatment decreased cholecystokinin-induced satiety. There was no alteration in leptin signaling in the hypothalamus. These findings offer a mechanism by which a change in gut microflora can promote hyperphagia, possibly leading to obesity.

Gut vagal afferents differentially modulate innate anxiety and learned fear

Vagal afferents are an important neuronal component of the gut-brain axis allowing bottom-up information flow from the viscera to the CNS. In addition to its role in ingestive behavior, vagal afferent signaling has been implicated modulating mood and affect, including distinct forms of anxiety and fear. Here, we used a rat model of subdiaphragmatic vagal deafferentation (SDA), the most complete and selective vagal deafferentation method existing to date, to study the consequences of complete disconnection of abdominal vagal afferents on innate anxiety, conditioned fear, and neurochemical parameters in the limbic system. We found that compared with Sham controls, SDA rats consistently displayed reduced innate anxiety-like behavior in three procedures commonly used in preclinical rodent models of anxiety, namely the elevated plus maze test, open field test, and food neophobia test. On the other hand, SDA rats exhibited increased expression of auditory-cued fear conditioning, which specifically emerged as attenuated extinction of conditioned fear during the tone re-exposure test. The behavioral manifestations in SDA rats were associated with region-dependent changes in noradrenaline and GABA levels in key areas of the limbic system, but not with functional alterations in the hypothalamus-pituitary-adrenal grand stress. Our study demonstrates that innate anxiety and learned fear are both subjected to visceral modulation through abdominal vagal afferents, possibly via changing limbic neurotransmitter systems. These data add further weight to theories emphasizing an important role of afferent visceral signals in the regulation of emotional behavior.

Pathophysiology and treatment of inflammatory anorexia in chronic disease

Decreased appetite and involuntary weight loss are common occurrences in chronic disease and have a negative impact on both quality of life and eventual mortality. Weight loss in chronic disease comes from both fat and lean mass, and is known as cachexia. Both alterations in appetite and body weight loss occur in a wide variety of diseases, including cancer, heart failure, renal failure, chronic obstructive pulmonary disease and HIV. An increase in circulating inflammatory cytokines has been implicated as a uniting pathogenic mechanism of cachexia and associated anorexia. One of the targets of inflammatory mediators is the central nervous system, and in particular feeding centers in the hypothalamus located in the ventral diencephalon. Current research has begun to elucidate the mechanisms by which inflammation reaches the hypothalamus, and the neural substrates underlying inflammatory anorexia. Research into these neural mechanisms has suggested new therapeutic possibilities, which have produced promising results in preclinical and clinical trials. This review will discuss inflammatory signaling in the hypothalamus that mediates anorexia, and the opportunities for therapeutic intervention that these mechanisms present.

Hypothalamic mechanisms in cachexia

The role of nutrition and balanced metabolism in normal growth, development, and health maintenance is well known. Patients affected with either acute or chronic diseases often show disorders of nutrient balance. In some cases, a devastating state of malnutrition known as cachexia arises, brought about by a synergistic combination of a dramatic decrease in appetite and an increase in metabolism of fat and lean body mass. Other common features that are not required for the diagnosis include decreases in voluntary movement, insulin resistance, and anhedonia. This combination is found in a number of disorders including cancer, cystic fibrosis, AIDS, rheumatoid arthritis, renal failure, and Alzheimer's disease. The severity of cachexia in these illnesses is often the primary determining factor in both quality of life, and in eventual mortality. Indeed, body mass retention in AIDS patients has a stronger association with survival than any other current measure of the disease. This has led to intense investigation of cachexia and the proposal of numerous hypotheses regarding its etiology. Most authors suggest that cytokines released during inflammation and malignancy act on the central nervous system to alter the release and function of a number of neurotransmitters, thereby altering both appetite and metabolic rate. This review will discuss the salient features of cachexia in human diseases, and review the mechanisms whereby inflammation alters the function of key brain regions to produce stereotypical illness behavior. The paper represents an invited review by a symposium, award winner or keynote speaker at the Society for the Study of Ingestive Behavior [SSIB] Annual Meeting in Portland, July 2009.

Neurobiology of inflammation-associated anorexia

Compelling data demonstrate that inflammation-associated anorexia directly results from the action of pro-inflammatory factors, primarily cytokines and prostaglandins E2, on the nervous system. For instance, the aforementioned pro-inflammatory factors can stimulate the activity of peripheral sensory neurons, and induce their own de novo synthesis and release into the brain parenchyma and cerebrospinal fluid. Ultimately, it results in the mobilization of a specific neural circuit that shuts down appetite. The present article describes the different cell groups and neurotransmitters involved in inflammation-associated anorexia and examines how they interact with neural systems regulating feeding such as the melanocortin system. A better understanding of the neurobiological mechanisms underlying inflammation-associated anorexia will help to develop appetite stimulants for cancer and AIDS patients.

Immune-to-brain signaling: how important are the blood-brain barrier-independent pathways?

A conceptual obstacle for understanding immune-to-brain signaling is the issue of the blood-brain barrier (BBB). In the last 30 years, several pathways have been investigated to address the question of how peripheral immune signals are transmitted into the brain. These pathways can be categorized into two types: BBB-dependent pathways and BBB-independent pathways. BBB-dependent pathways involve the BBB as a relay station or porous barrier, whereas BBB-independent pathways use neuronal routes that bypass the BBB. Recently, a complete BBB-dependent ascending pathway for immune-to-brain signaling has been described. Details of BBB-independent pathways are still under construction. In this review, I will summarize the current progress in unraveling immune-to-brain signaling pathways. In addition, I will provide a critical analysis of the literature to point to areas where our knowledge of the immunological afferent signaling to the central nervous system is still sorely lacking.

IL-1beta and LPS induce anorexia by distinct mechanisms differentially dependent on microsomal prostaglandin E synthase-1

Recent work demonstrated that the febrile response to peripheral immune stimulation with proinflammatory cytokine IL-1beta or bacterial wall lipopolysaccharide (LPS) is mediated by induced synthesis of prostaglandin E(2) by the terminal enzyme microsomal prostaglandin E synthase-1 (mPGES-1). The present study examined whether a similar mechanism might also mediate the anorexia induced by these inflammatory agents. Transgenic mice with a deletion of the Ptges gene, which encodes mPGES-1, and wild-type controls were injected intraperitoneally with IL-1beta, LPS, or saline. Mice were free fed, and food intake was continuously monitored with an automated system for 12 h. Body weight was recorded every 24 h for 4 days. The IL-1beta induced anorexia in wild-type but not knock-out mice, and so it was almost completely dependent on mPGES-1. In contrast, LPS induced anorexia of the same magnitude in both phenotypes, and hence it was independent of mPGES-1. However, when the mice were prestarved for 22 h, LPS induced anorexia and concomitant body weight loss in the knock-out animals that was attenuated compared with the wild-type controls. These data suggest that IL-1beta and LPS induce anorexia by distinct immune-to-brain signaling pathways and that the anorexia induced by LPS is mediated by a mechanism different from the fever induced by LPS. However, nutritional state and/or motivational factors also seem to influence the pathways for immune signaling to the brain. Furthermore, both IL-1beta and LPS caused reduced meal size but not meal frequency, suggesting that both agents exerted an anhedonic effect during these experimental conditions.

Effect of subdiaphragmatic vagotomy on the noradrenergic and HPA axis activation induced by intraperitoneal interleukin-1 administration in rats

The vagus nerve is thought to participate in signal transduction from the immune system to the CNS. The role of the vagus in the physiological, behavioral and neurochemical responses to intraperitoneally (ip) injected interleukin-1beta (IL-1beta) was studied using awake subdiaphragmatically vagotomized rats. The rats were injected ip with saline and IL-1beta (1 microg/rat) in random order. For the next 2-4 h, they were monitored for locomotor activity, body temperature via abdominally implanted telethermometers, hypothalamic norepinephrine (NE) secretion using in vivo microdialysis and blood sampled via intravenous catheters to determine concentrations of ACTH and corticosterone to assess hypothalamo-pituitary-adrenocortical (HPA) axis activation. Saline injections were followed by transient increases in locomotor activity, body temperature, dialysate NE and plasma concentrations of ACTH and corticosterone. These responses were not significantly altered by vagotomy. IL-1beta injections resulted in short-lived increases in shivering and longer decreases in locomotor activity, as well as a delayed modest fever. IL-1beta also induced prolonged elevations of hypothalamic microdialysate NE, as well as plasma ACTH and corticosterone. Similar responses were observed regardless of the order of the saline and IL-1beta injections. Subdiaphragmatic vagotomy prevented the IL-1-induced increases in body temperature and the increase in dialysate NE, and markedly attenuated the increases in plasma ACTH and corticosterone. The results indicate close temporal relationships between the apparent release of NE and the increase in body temperature and the HPA activation. This together with the effects of vagotomy suggests that the activation of NE in turn increases body temperature and activates the HPA axis. However, because IL-1beta induces a limited HPA activation in subdiaphragmatically vagotomized rats, the vagus nerve does not appear to be the only route by which ip IL-1beta can activate the HPA axis. It is suggested that IL-1beta-induced vagal activation of hypothalamic NE is the major mechanism of HPA activation at low doses of IL-1beta. However, IL-1beta can also exert direct effects on IL-1 receptors on cerebral blood vessels, activating cyclooxygenases and hence synthesis of prostaglandins which in turn can affect body temperature, behavior and HPA axis activation.

Physiological and behavioral responses to interleukin-1beta and LPS in vagotomized mice

It is well established that peripheral administration of interleukin-1 (IL-1) and lipopolysaccharide (LPS) can activate the hypothalamo-pituitary-adrenocortical (HPA) axis, alter brain catecholamine and indoleamine metabolism, and affect behavior. However, the mechanisms of these effects are not fully understood. Stimulation of afferents of the vagus nerve has been implicated in the induction of Fos in the brain, changes in body temperature, brain norepinephrine, and some behavioral responses. In the present study, the IL-1beta- and LPS-induced changes in certain behaviors, HPA axis activation, and catecholamine and indoleamine metabolism were studied in mice following subdiaphragmatic vagotomy. IL-1beta and LPS induced the expected decreases in sweetened milk, food intake, and locomotor activity, and the responses to IL-1beta, but not LPS, were slightly attenuated in vagotomized mice. Subdiaphragmatic vagotomy also attenuated the IL-1beta- and LPS-induced increases in plasma ACTH and corticosterone, but the attenuations of the responses to IL-1beta were only marginally significant. There were also slight reductions in the responses in catecholamine and serotonin metabolism, and the increases in brain tryptophan in several brain regions. These results indicate that the vagus nerve is not the major pathway by which abdominal IL-1beta and LPS effect behavioral, HPA and brain catecholamine and indoleamine responses in the mouse. These results resemble those we observed in subdiaphragmatically vagotomized rats, but in that species the subdiaphragmatic vagotomy markedly attenuated the ACTH and corticosterone responses, and prevented the hypothalamic noradrenergic activation, as well as the fever. Overall the results indicate that the various responses to peripheral IL-1 and LPS involve multiple mechanisms including vagal afferents, and that there are species differences in the relative importance of the various mechanisms.

Celiac-superior mesenteric ganglionectomy, but not vagotomy, suppresses the sympathoadrenal response to insulin-induced hypoglycemia

Afferent innervation of the portal vein has been shown to be critical in hypoglycemic detection, but the neural pathway by which these afferents ascend remains unknown. To ascertain the role of vagal afferents versus spinal afferents in hypoglycemic detection, the catecholamine response to hypoglycemia was assessed in male Wistar rats undergoing hepatic vagotomy (HV), total subdiaphragmatic vagotomy (TSV), or celiac-superior mesenteric ganglionectomy (CSMG). After recovering from the surgery, the animals were exposed to a hyperinsulinemic-hypoglycemic clamp, with glucose infused peripherally via the jugular vein. In all animals, systemic hypoglycemia (2.64 +/- 0.03 mmol/l) was induced via jugular vein insulin infusion (25 mU x kg(-1) x min(-1)). No significant differences were observed among the groups with respect to arterial glucose or insulin concentration. When hypoglycemia was induced in sham-operated control animals, epinephrine was observed to rise from a basal value of 0.84 +/- 0.10 to 25.18 +/- 1.24 nmol/l. Neither HV nor TSV had any significant impact on the epinephrine response to hypoglycemia. In contrast, CSMG animals demonstrated a significant suppression in the epinephrine response to whole-body hypoglycemia (11.25 +/- 1.21 vs. 22.32 +/- 0.86 nmol/l in CSMG vs. controls; P < 0.05). The norepinephrine response for controls, 2.00 +/- 0.22 at basal and rising to 8.95 +/- 0.20 nmol/l in hypoglycemia, was not significantly different from that of the HV and TSV animals. As with epinephrine, the norepinephrine response to hypoglycemia was significantly suppressed in CSMG compared with control animals (4.72 +/- 0.48 vs. 7.15 +/- 0.76 nmol/l; P < 0.05). These findings are consistent with the idea that hypoglycemic detection at the portal vein is mediated by spinal, and not vagal, glucose-sensitive afferents.

Thoracic cross-over pathways of the rat vagal trunks

It is very difficult to study the independent contributions of the afferent and efferent pathways of the subdiaphragmatic vagus to physiology and behavior. Total subdiaphragmatic vagotomy can confound the interpretation of experimental results because it destroys both afferent and efferent vagal fibers. One approach to address this problem involves producing a total ablation of afferent (or efferent) vagal fibers while retaining half of the efferent (or afferent) vagal fibers by making a unilateral rhizotomy plus contralateral subdiaphragmatic vagotomy. However, the completeness of this afferent (or efferent) lesion is based on the assumption that there are no cross-over pathways within the thoracic cavity between the vagal trunks of the rat. To directly test for the presence of vagal cross-over pathways in the rat, we recorded the compound action potentials from the ventral and dorsal trunks of the subdiaphragmatic vagus following electrical stimulation of the left or right cervical vagi. C-fiber cross-over pathways comprised an average of 9% of the total nerve responses (range was 0 to 29%, n = 20). Direct application of the anesthetic bupivacaine to the vagus completely blocked the recorded signals. The vagal cross-over pathways were also demonstrated using capsaicin as a stimulus. These results indicate the presence of thoracic cross-over pathways between vagal trunks in the rat and demonstrate that for most animals it is not possible to produce a "complete" ablation of afferent (or efferent) components of the subdiaphragmatic vagus using unilateral rhizotomy combined with contralateral subdiaphragmatic vagotomy.

Circulating cytokines and endotoxin are not necessary for the activation of the sickness or corticosterone response produced by peripheral E. coli challenge

Peripheral administration of a variety of inflammatory stimuli, such as endotoxin or cytokines, induces an orchestrated set of brain-mediated events referred to as the sickness response. The mechanism for how immune products signal the brain is not clear, but accumulating evidence supports the existence of neural as well as blood-borne pathways. Although endotoxin or cytokine administration results in sickness responses, few data exist regarding the role of circulating endotoxin or cytokines in the induction of sickness during a real bacterial infection. Thus the present studies examined whether subcutaneously administered Escherichia coli can activate sickness responses and whether circulating endotoxin and/or proinflammatory cytokines are a prerequisite for these responses. Male Sprague-Dawley rats were injected subcutaneously with one of three doses (2.5 x 10(7), 2.5 x 10(8), 2.5 x 10(9) colony-forming units) of replicating E. coli, a ubiquitous bacterial strain, or vehicle. Core body temperature (Tc) and activity were measured for 3 days after the injection. A second set of groups of animals were killed 3, 6, 12, 18, 24, and 48 h after the injection, and blood samples and brains were collected. Injections dose dependently and consistently increased Tc and decreased activity, with increases in Tc beginning 4 h after the injection. In addition, E. coli significantly increased serum interleukin (IL)-1beta, IL-6, and tumor necrosis factor-alpha and brain IL-1beta levels beginning at the 6-h time point. Corticosterone and endotoxin were first elevated in the circulation at 3 and 18 h after the injection, respectively. Because fever onset preceded brain cytokine induction, we also examined cytokine levels in the serum, brain, and inflammation site 2 and 4 h after injection. Cytokines were elevated at the inflammation site but were not detectable in the serum or brain at 2 and 4 h. We conclude that subcutaneous injection of replicating E. coli induces a consistent and naturalistic infection that includes features of the sickness response as well as increases in circulating, brain, and inflammation site tissue cytokines. In addition, injection of replicating E. coli produces a robust fever and corticosterone response at a time when there are no detectable increases in circulating cytokines or endotoxin. These results suggest that elevated levels of circulating cytokines and endotoxin are not necessary for the activation of the sickness or corticosterone response. Therefore, fever, activity reduction, and corticosterone elevation induced by E. coli infection may have been evoked by a neural, rather than a humoral, pathway from the periphery to the brain.

A role for cyclooxygenase-2 in lipopolysaccharide-induced anorexia in rats

Because nonselective cycloooxygenase (COX) inhibition attenuated anorexia after lipopolysaccharide (LPS) administration, we tested the ability of resveratrol (2.5, 10, and 40 mg/kg) and NS-398 (2.5, 10, and 40 mg/kg), selective inhibitors of the two COX isoforms COX-1 and -2, respectively, to attenuate LPS (100 microg/kg ip)-induced anorexia. NS-398 (10 and 40 mg/kg) administered with LPS at lights out attenuated LPS-induced anorexia, whereas resveratrol at all doses tested did not. Because prostaglandin (PG) E(2) is considered the major metabolite synthesized by COX, we measured plasma and cerebrospinal fluid (CSF) PGE(2) levels after LPS administration. LPS induced a time-dependent increase of PGE(2) in CSF but not in plasma. NS-398 (5, 10, and 40 mg/kg) blocked the LPS-induced increase in CSF PGE(2), whereas resveratrol (10 mg/kg) did not. These results support a role of COX-2 in mediating the anorectic response to peripheral LPS and point at PGE(2) as a potential neuromodulator involved in this response.

Subdiaphragmatic vagotomy fails to inhibit intravenous leptin-induced IL-1beta expression in the hypothalamus

Leptin is known to be an important circulating signal for regulation of food intake and body weight. Recent evidence has suggested that leptin is involved in infection and inflammation. The afferent vagus nerve is known to be an important component for transmitting peripheral immune signals to the brain, such as interleukin (IL)-1beta expression in the brain, anorexia, and fever responses. In the present study, we investigated whether intravenous leptin-induced IL-1beta expression in the hypothalamus is mediated via afferent vagus nerve. IL-1beta transcripts in the hypothalamus were significantly increased on RT-PCR assessment 1 h after the administration of leptin (1 mg/kg iv) to mice. Subdiaphragmatic vagotomy did not significantly modify intravenous leptin-induced IL-1beta expression in the hypothalamus compared with that in sham-treated mice. These data suggest that circulating leptin directly acts in the brain independently of afferent vagus nerve input originating from the subdiaphragmatic organs.

TNF-alpha-induced c-Fos generation in the nucleus of the solitary tract is blocked by NBQX and MK-801

Previous studies have shown that identified neurons of the nucleus of the solitary tract (NST) are excited by the cytokine tumor necrosis factor-alpha (TNF-alpha). Vagal afferent connections with the NST are predominantly glutaminergic. Therefore, we hypothesized that TNF-alpha effects on NST neurons may be via modulation of glutamate neurotransmission. The present study used activation of the immediate early gene product c-Fos as a marker for neuronal activation in the NST. c-Fos expression was evaluated after microinjections of TNF-alpha in the presence or absence of either the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor antagonist 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide disodium (NBQX) or the N-methyl-D- aspartate (NMDA) antagonist MK-801. To assess the specificity of the interaction between TNF-alpha and glutamate, c-Fos expression was also evaluated after injection of oxytocin (OT) (which has a direct excitatory effect in this area of the brain stem) in the presence and absence of NBQX or MK-801. c-Fos labeling was significantly increased in the NST after TNF-alpha exposure. Coinjection of either NBQX or MK-801 with TNF-alpha prevented significant c-Fos induction in the NST. Microinjections of OT also induced significant NST c-Fos elevation, but this expression was unaffected by coinjection of either antagonist with OT. These data lead us to conclude that TNF-alpha activation of NST neurons depends on glutamate and such an interaction is not generalized to all agonists that act on the NST.

The contribution of the vagus nerve in interleukin-1beta-induced fever is dependent on dose

It has been suggested that proinflammatory cytokines communicate to the brain via a neural pathway involving activation of vagal afferents by interleukin-1beta (IL-1beta), in addition to blood-borne routes. In support, subdiaphragmatic vagotomy blocks IL-1beta-induced, brain-mediated responses such as fever. However, vagotomy has also been reported to be ineffective. Neural signaling would be expected to be especially important at low doses of cytokine, when local actions could occur, but only very small quantities of cytokine would become systemic. Here, we examined core body temperature after intraperitoneal injections of three doses of recombinat human IL-1beta (rh-IL-1beta). Subdiaphragmatic vagotomy completely blocked the fever produced by 0.1 microg/kg, only partially blocked the fever produced by 0.5 microg/kg, and had no effect at all on the fever that followed 1.0 microg/kg rh-IL-1beta. Blood levels of rh-IL-1beta did not become greater than normal basal levels of endogenous rat IL-beta until the 0.5-microg/kg dose nor was IL-1beta induced in the pituitary until this dose. These results suggest that low doses of intraperitoneal IL-1beta induce fever via a vagal route and that dose may account for some of the discrepancies in the literature.

c-Fos generation in the dorsal vagal complex after systemic endotoxin is not dependent on the vagus nerve

The present study used activation of the c-Fos oncogene protein within neurons in the dorsal vagal complex (DVC) as a marker of neuronal excitation in response to systemic endotoxin challenge [i.e. , lipopolysaccharide (LPS)]. Specifically, we investigated whether vagal connections with the brain stem are necessary for LPS cytokine- induced activation of DVC neurons. Systemic exposure to LPS elicited a significant activation of c-Fos in neurons in the nucleus of the solitary tract (NST) and area postrema of all thiobutabarbital-anesthetized rats examined, regardless of the integrity of their vagal nerves. That is, rats with both vagi cervically transected were still able to respond with c-Fos activation of neurons in the DVC. Unilateral cervical vagotomy produced a consistent but small reduction in c-Fos activation in the ipsilateral NST of all animals within this experimental group. Given that afferent input to the NST is exclusively excitatory, it is not surprising that unilateral elimination of all vagal afferents would diminish NST responsiveness (on the vagotomized side). These data lead us to conclude that the NST itself is a primary central nervous system detector of cytokines.

Electrical stimulation of afferent vagus nerve induces IL-1beta expression in the brain and activates HPA axis

Possible roles of the afferent vagus nerve in regulation of interleukin (IL)-1beta expression in the brain and hypothalamic-pituitary-adrenal (HPA) axis were examined in anesthetized rats. Levels of IL-1beta mRNA and protein in the brain were measured by comparative RT-PCR and ELISA. Direct electrical stimulation of the central end of the vagus nerve was performed continuously for 2 h. The afferent stimulation of the vagus nerve induced increases in the expression of mRNA and protein levels of IL-1beta in the hypothalamus and the hippocampus. Furthermore, expression of corticotropin-releasing factor mRNA was increased in the hypothalamus 2 h after vagal stimulation. Plasma levels of ACTH and corticosterone were also increased by this stimulation. The present results indicate that activation of the afferent vagus nerves itself can induce production of IL-1beta in the brain and activate the HPA axis. Therefore, the afferent vagus nerve may play an important role in transmitting peripheral signals to the brain in the infection and inflammation.