Behavioral conditioning of lipopolysaccharide-induced anorexia

A translational and multidisciplinary approach to studying the Garcia effect, a higher form of learning with deep evolutionary roots

Animals, including humans, learn and remember to avoid a novel food when its ingestion is followed, hours later, by sickness - a phenomenon initially identified during World War II as a potential means of pest control. In the 1960s, John Garcia (for whom the effect is now named) demonstrated that this form of conditioned taste aversion had broader implications, showing that it is a rapid but long-lasting taste-specific food aversion with a fundamental role in the evolution of behaviour. From the mid-1970s onward, the principles of the Garcia effect were translated to humans, showing its role in different clinical conditions (e.g. side-effects linked to chemotherapy). However, in the last two decades, the number of studies on the Garcia effect has undergone a considerable decline. Since its discovery in rodents, this form of learning was thought to be exclusive to mammals; however, we recently provided the first demonstration that a Garcia effect can be formed in an invertebrate model organism, the pond snail Lymnaea stagnalis. Thus, in this Commentary, after reviewing the experiments that led to the first characterization of the Garcia effect in rodents, we describe the recent evidence for the Garcia effect in L. stagnalis, which may pave the way for future studies in other invertebrates and mammals. This article aims to inspire future translational and ecological studies that characterize the conserved mechanisms underlying this form of learning with deep evolutionary roots, which can be used to address a range of different biological questions.

Pavlovian Conditioning of Immunological and Neuroendocrine Functions

The phenomenon of behaviorally conditioned immunological and neuroendocrine functions has been investigated for the past 100 yr. The observation that associative learning processes can modify peripheral immune functions was first reported and investigated by Ivan Petrovic Pavlov and his co-workers. Their work later fell into oblivion, also because so little was known about the immune system’s function and even less about the underlying mechanisms of how learning, a central nervous system activity, could affect peripheral immune responses. With the employment of a taste-avoidance paradigm in rats, this phenomenon was rediscovered 45 yr ago as one of the most fascinating examples of the reciprocal functional interaction between behavior, the brain, and peripheral immune functions, and it established psychoneuroimmunology as a new research field. Relying on growing knowledge about efferent and afferent communication pathways between the brain, neuroendocrine system, primary and secondary immune organs, and immunocompetent cells, experimental animal studies demonstrate that cellular and humoral immune and neuroendocrine functions can be modulated via associative learning protocols. These (from the classical perspective) learned immune responses are clinically relevant, since they affect the development and progression of immune-related diseases and, more importantly, are also inducible in humans. The increased knowledge about the neuropsychological machinery steering learning and memory processes together with recent insight into the mechanisms mediating placebo responses provide fascinating perspectives to exploit these learned immune and neuroendocrine responses as supportive therapies, the aim being to reduce the amount of medication required, diminishing unwanted drug side effects while maximizing the therapeutic effect for the patient’s benefit.

Lipopolysacharide Rapidly and Completely Suppresses AgRP Neuron-Mediated Food Intake in Male Mice

Although Agouti-related peptide (AgRP) neurons play a key role in the regulation of food intake, their contribution to the anorexia caused by proinflammatory insults has yet to be identified. Using a combination of neuroanatomical and pharmacogenetics experiments, this study sought to investigate the importance of AgRP neurons and downstream targets in the anorexia caused by the peripheral administration of a moderate dose of lipopolysaccharide (LPS) (100 μg/kg, ip). First, in the C57/Bl6 mouse, we demonstrated that LPS induced c-fos in select AgRP-innervated brain sites involved in feeding but not in any arcuate proopiomelanocortin neurons. Double immunohistochemistry further showed that LPS selectively induced c-Fos in a large subset of melanocortin 4 receptor-expressing neurons in the lateral parabrachial nucleus. Secondly, we used pharmacogenetics to stimulate the activity of AgRP neurons during the course of LPS-induced anorexia. In AgRP-Cre mice expressing the designer receptor hM3Dq-Gq only in AgRP neurons, the administration of the designer drug clozapine-N-oxide (CNO) induced robust food intake. Strikingly, CNO-mediated food intake was rapidly and completely blunted by the coadministration of LPS. Neuroanatomical experiments further indicated that LPS did not interfere with the ability of CNO to stimulate c-Fos in AgRP neurons. In summary, our findings combined together support the view that the stimulation of select AgRP-innervated brain sites and target neurons, rather than the inhibition of AgRP neurons themselves, is likely to contribute to the rapid suppression of food intake observed during acute bacterial endotoxemia.

Single-trial conditioning in a human taste-endotoxin paradigm induces conditioned odor aversion but not cytokine responses

Immunological responses to bacterial endotoxin can be behaviorally conditioned in rodents. However, it is unclear whether an acute systemic inflammatory response can be behaviorally conditioned in humans. Thus, in a double-blind placebo-controlled study, 20 healthy, male subjects received either a single injection of lipopolysaccharide (LPS) or saline together with a novel tasting beverage (conditioned stimulus, CS). Five days later, all subjects received a saline injection and were re-exposed to the CS. Blood was drawn prior to as well as 0.5, 1.5, 3, 4, 6, and 24 h after LPS administration or CS re-exposure. Endotoxin administration led to transient increases in plasma concentrations of interleukin (IL)-6, IL-10, and tumor necrosis factor (TNF)-α and to a significant rise in body temperature. Sole presentation of the CS during evocation did induce neither alterations in body temperature nor changes in plasma cytokine levels. However, subjects in the experimental group rated the smell of the CS significantly more aversive compared to the control group. Employing endotoxin as a US in a single trial taste-immune conditioning paradigm in humans shows a behaviorally conditioned smell aversion but no learned alterations in cytokine levels.

Electrical activity in rat cortico-limbic structures after single or repeated administration of lipopolysaccharide or staphylococcal enterotoxin B

Immune-to-brain communication is essential for an individual to aptly respond to challenging internal and external environments. However, the specificity by which the central nervous system detects or 'senses' peripheral immune challenges is still poorly understood. In contrast to post-mortem c-Fos mapping, we recorded neural activity in vivo in two specific cortico-limbic regions relevant for processing visceral inputs and associating it with other sensory signalling, the amygdala (Am) and the insular cortex (IC). Adult rats were implanted with deep-brain monopolar electrodes and electrical activity was monitored unilaterally before and after administration of two different immunogens, the T-cell-independent antigen lipopolysaccharide (LPS) or the T-cell-dependent antigen staphylococcal enterotoxin B (SEB). In addition, the neural activity of the same individuals was analysed after single as well as repeated antigen administration, the latter inducing attenuation of the immune response. Body temperature and circulating cytokine levels confirmed the biological activity of the antigens and the success of immunization and desensitization protocols. More importantly, the present data demonstrate that neural activity of the Am and IC is not only specific for the type of immune challenge (LPS versus SEB) but seems to be also sensitive to the different immune state (naive versus desensitization). This indicates that the forebrain expresses specific patterns of electrical activity related to the type of peripheral immune activation as well as to the intensity of the stimulation, substantiating associative learning paradigms employing antigens as unconditioned stimuli. Overall, our data support the view of an intensive immune-to-brain communication, which may have evolved to achieve the complex energetic balance necessary for mounting effective immunity and improved individual adaptability by cognitive functions.

The learned immune response: Pavlov and beyond

The ability to associate physiological changes with a specific flavor was most likely acquired during evolution as an adaptive strategy aimed at protecting the organism while preparing it for danger. The behaviorally conditioned or learned immune response is an exquisite example of the bidirectional communication between the central nervous system (CNS) and the peripheral immune system. How is it possible that specific immuno-modulating properties of a drug or substance (unconditioned stimulus) can be re-enlisted just by the mere re-exposure to a particular taste, odor or environment (conditioned stimulus)? To answer this key question, we review the neurobiological mechanism mediating this type of associative learning, as well as the pathways and mechanisms employed by the brain to harness the immune system during the execution of the conditioned immune response. Finally, we focus on the potential therapeutic relevance of such learned immune responses, and their re-conceptualization within the framework of "learned placebo effects".

Signals generating anorexia during acute illness

Anorexia is part of the body's acute-phase response to illness. Microbial products such as lipopolysaccharides (LPS), which are also commonly used to model acute illness, trigger the acute-phase response and cause anorexia mainly through pro-inflammatory cytokines. LPS stimulate cytokine production through the cell-surface structural molecule CD14 and toll-like receptor-4. Cytokines ultimately change neural activity in brain areas controlling food intake and energy balance. The blood-brain barrier endothelial cells (BBB EC) are an important site of cytokine action in this context. BBB EC and perivascular cells (microglia and macrophages) form a complex regulatory interface that modulates neuronal activity by the release of messengers (e.g. PG, NO) in response to peripheral challenges. Serotonergic neurons originating in the raphe nuclei and glucagon-like peptide-1-expressing neurons in the hindbrain may be among the targets of these messengers, because serotonin (5-HT), acting through the 5-HT2C receptor, and glucagon-like peptide-1 have recently emerged as neurochemical mediators of LPS anorexia. The central melanocortin system, which is a downstream target of serotonergic neurons, also appears to be involved in mediation of LPS anorexia. Interestingly, LPS also reduce orexin expression and the activity of orexin neurons in the lateral hypothalamic area of fasted mice. As the eating-stimulatory properties of orexin are apparently related to arousal, the inhibitory effect of LPS on orexin neurons might be involved in LPS-induced inactivity and anorexia. In summary, the immune signalling pathways of LPS-induced, and presumably acute illness-induced, anorexia converge on central neural signalling systems that control food intake and energy balance in healthy individuals.

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.

Central blockade of IL-1 does not impair taste-LPS associative learning

After saccharin intake is associated with the consequences of peripheral lipopolysaccharide (LPS) administration, rats develop a strong conditioned avoidance behavior against this gustatory stimulus. To investigate the role of central interleukin-1 (IL-1) as a key signal during taste-LPS engram formation, rats were chronically infused with IL-1 receptor antagonist into the lateral ventricle of the brain before, during and after a single association trial. The results indicate that a stable taste-LPS engram can be formed even under the chronic blockade of central IL-1 signaling during engram formation and consolidation. More importantly, our data show that animals which did not experience a fever response during association phase (due to the LPS encounter) were unable to elicit hyperthermia as part of the conditioned response. These data indicate that pairing a relevant taste stimulus with an immune challenge, such as LPS, might result in the formation of multiple engrams, specifically codifying independent information.

Maternal exposure to LPS induces hypomyelination in the internal capsule and programmed cell death in the deep gray matter in newborn rats

Epidemiologic and experimental findings implicate maternal infection in the etiology of injury to brain white matter, which may lead to cerebral palsy in preterm newborns. In the present study, inflammation and brain damage in 1- and 7-d-old rats were investigated after maternal inflammation. Intraperitoneal injection of 300 microg/kg of Escherichia coli lipopolysaccharide was administered to pregnant Wistar rats at d 19 and 20 of gestation (LPS group). Control females received a saline injection. Proinflammatory cytokines IL-1beta, tumor necrosis factor-alpha, and IL-6 expression in the fetal brain were determined by reverse transcription quantitative polymerase chain reaction. Brain injury was examined in 16-mum coronal brain sections by GFAP, MBP, caspase-3 immunohistochemistry, and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling. Expression of IL-1beta was significantly increased 3 d after maternal administration (P1). A significant increase in cell death occurred at P1 and P7 in specific brain areas, i.e. in the subventricular striatal zone at P1, and in 1) the periventricular striatum, 2) the periventricular white matter, and 3) the germinative ventricular zone at P7. We also observed typical astrogliosis and strong hypomyelination in the external and internal capsule in the LPS group at P7. These results demonstrate that maternal LPS treatment induces persistent fetal inflammatory reactions associated with significant white matter injury in progeny at P1 and P7. This model should be relevant for the study of the pathophysiological mechanisms involved in cerebral white matter damage in preterm human newborns and in the development of therapeutic strategies.

Behavioral conditioning with interferon beta-1a in humans

Behavioral conditioning is one of the most impressive demonstrations of brain-immune system interaction. Numerous animal studies have demonstrated behavioral conditioned effects on immune functions, however, human studies are rare. We investigated whether it is possible to behaviorally condition the acute response to interferon (IFN)beta-1a. In a double-blind placebo-controlled study, 30 healthy subjects received a single injection of IFN(beta)-1a (6MIU of REBIF, Serono International) (unconditioned stimulus, UCS) together with a novel drink (conditioned stimulus, CS). Blood was drawn at baseline, 4, 8, and 24 h after drug administration. Within the first 8 h peripheral granulocytes significantly increased, while monocytes, lymphocytes, T-, B- and natural killer (NK) cell numbers were significantly reduced. In parallel, body temperature, heart rate, norepinephrine and interleukin (IL)-6 plasma levels were heightened within 8 h after injection. 8 days later, all previously IFN(beta)-treated subjects received a subcutaneous placebo (NaCl) injection, but only 15 subjects were re-exposed to the CS (experimental group), while a control group (N=15) drank water and an additional group of subjects (n=8) remained untreated (untreated group). Blood sampling was performed at baseline and at 4, 8, and 24 h. Re-exposition to the CS did not elicit conditioned responses in the experimental group. Moreover, no differences were observed between groups. These data provide negative findings regarding behavioral conditioning of cytokine effects in humans employing a one-trial learning paradigm.

Self-rated health is related to levels of circulating cytokines

OBJECTIVE

Self-rated health is a powerful and independent predictor of long-term health, but its biological basis is unknown. Because factors associated with poor self-rated health (eg, pain, daily discomforts, and low energy and fitness) resemble symptoms of a generalized cytokine-induced sickness response, we examined the relationship between circulating cytokines and self-rated health.

METHODS

In 265 consecutive primary health care patients (174 women and 91 men), we examined self-rated and physician-rated health, circulating levels of interleukin (IL)-1beta, IL-1 receptor antagonist (IL-1ra), IL-6, and tumor necrosis factor (TNF)-alpha as determined from plasma samples using high-sensitivity enzyme-linked immunoassay.

RESULTS

Self-rated health correlated with levels of IL-1beta (r = 0.27; p <.001), IL-1ra (r = 0.19; p <.05) and TNF-alpha (r = 0.46; p <.001) in women but not in men. Thus, poorer subjective health was associated with higher levels of inflammatory cytokines. Even when controlling for age, education, physical health, and diagnoses in multiple regression analyses, self-rated health was an independent and more robust predictor of cytokine levels than physician-rated health.

CONCLUSIONS

The present findings suggest that an individual's health perception may be coupled to circulating cytokines. Because epidemiological research established that self-rated health predicts morbidity and mortality, the biological correlates and mechanisms of self-rated health need to be understood.

Conditioned taste aversion with lipopolysaccharide and peptidoglycan does not activate cytokine gene expression in the spleen and hypothalamus of mice

Several reports show that behavioural and physiological components of the acute phase reaction can be conditioned. However, the mechanisms responsible for these effects remain obscure. The underlying assumption that the changes observed in conditioned animals are dependent on a conditioned production of cytokines has never been demonstrated. In the present study, the possibility of conditioning the production of cytokines or molecules implicated in their signalling pathways was tested by submitting mice to conditioned taste aversion with a new saccharin taste paired with intraperitoneal (i.p.) injections of lipopolysaccharide (LPS, 0.83 microg/g) or peptidoglycan (PGN, 20 microg/g). After two conditioning sessions, conditioned mice developed a clear aversion to saccharine that was not associated with activation of genes of the cytokine network either at the periphery, or in the hypothalamus, as demonstrated by a macroarray approach and confirmed by real time RT-PCR. In contrast, there was an activation of the genes coding for nuclear factor kappa B (NFkappaB) and mitogen activated protein kinase (MAPK) signalling pathways in the spleen and to a lesser extent in the hypothalamus. This modulation of the NFkappaB and MAPK signalling pathways is interpreted in terms of a possible conditioned sensitisation of the immune system.

Conditioned place aversion with interleukin-1beta in mice is not associated with activation of the cytokine network

Several distinct findings argue in favor of conditioning of some components of the acute phase reaction. However, the possibility of a conditioned cytokine response has not been assessed. In the present study, this possibility was tested by submitting mice to place aversion conditioning with interleukin-1beta (2 microgram/mouse, ip) as the unconditioned stimulus and an odorous compartment of a two-compartment cage as the conditioned stimulus. After two pairings, conditioned mice developed place aversion towards the odorous compartment. However, this behavioral conditioning was not accompanied by any alteration in peripheral (spleen) and brain (hypothalamus) cytokine levels (interleukin-1, interleukin-6, and interleukin-10). These data do not support the possibility of conditioned alterations in the cytokine network.

Pavlovian conditioning of endotoxin-tolerance in rats

The most fascinating example of the bi-directional interaction between the central nervous system (CNS) and immune system is the behavioral conditioning of immune functions. We therefore investigated the behavioral conditioning of lipopolysaccharide (LPS)-induced endotoxin tolerance using the taste aversion paradigm. The conditioned stimulus (CS) saccharin was paired with the unconditioned stimulus (UCS) LPS over a five (CONDl) or four (COND2) days learning trial. Controls received drinking water with (SHAM) or without (UNT) LPS. Endotoxin tolerance was tested by determination of LPS-induced tumor necrosis factor (TNF)-alpha release. After the avoidance of the induced endotoxin-tolerance the CS saccharin was re-presented in all experimental groups. A the end of the re-exposure period a complete endotoxin tolerance was noticed in the CONDl- and COND2-group. In contrast, no effect of saccharin administration was observed in the SHAM- or UNT-group. Our data demonstrate for the first time the behavioral conditioning of endotoxin tolerance. Furthermore, these results contribute new aspects to the mechanisms underlying the development and modulation of endotoxin tolerance.

Cytokine-induced sickness behavior: mechanisms and implications

Sickness behavior refers to a coordinated set of behavioral changes that develop in sick individuals during the course of an infection. At the molecular level, these changes are due to the brain effects of proinflammatory cytokines such as interleukin-1 (IL-1) and tumor necrosis factor alpha (TNFalpha). Peripherally released cytokines act on the brain via a fast transmission pathway involving primary afferent nerves innervating the bodily site of inflammation and a slow transmission pathway involving cytokines originating from the choroid plexus and circumventricular organs and diffusing into the brain parenchyma by volume transmission. At the behavioral level, sickness behavior appears to be the expression of a central motivational state that reorganizes the organism priorities to cope with infectious pathogens. There is evidence that the sickness motivational state can interact with other motivational states and respond to nonimmune stimuli probably by way of sensitization and/or classical conditioning. However, the mechanisms that are involved in plasticity of the sickness motivational state are not yet understood.

Cytokine-induced sickness behavior: where do we stand?

Sickness behavior refers to the coordinated set of behavioral changes that develop in sick individuals during the course of an infection. At the molecular level, these changes are due to the effects of proinflammatory cytokines, such as interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNFalpha), in the brain. Peripherally released cytokines act on the brain via a fast transmission pathway involving primary afferent nerves innervating the body site of inflammation and a slow transmission pathway involving cytokines originating from the choroid plexus and circumventricular organs and diffusing into the brain parenchyma by volume transmission. At the behavioral level, sickness behavior appears to be the expression of a central motivational state that reorganizes the organism's priorities to cope with infectious pathogens. There is clinical and experimental evidence that activation of the brain cytokine system is associated with depression, although the exact relationship between sickness behavior and depression is still elusive.

Anorexia of infection: current prospects

The anorexia of infection is part of the host's acute phase response (APR). Despite being beneficial in the beginning, long lasting anorexia delays recovery and is ultimately deleterious. Microbial products such as bacterial cell wall compounds (e.g., lipopolysaccharides and peptidoglycans), microbial nucleic acids (e. g., bacterial DNA and viral double-stranded RNA), and viral glycoproteins trigger the APR and presumably also the anorexia during infections. Microbial products stimulate the production of proinflammatory cytokines (e.g., interleukins [ILs], tumor necrosis factor-alpha, interferons), which serve as endogenous mediators. Several microbial products and cytokines reduce food intake after parenteral administration, suggesting a role of these substances in the anorexia during infection. Microbial products are mainly released and cytokines are produced in the periphery during most infections; they might inhibit feeding through neural and humoral pathways activated by their peripheral actions. Activation of peripheral afferents by locally produced cytokines is involved in several cytokine effects, but is not crucial for the anorectic effect of microbial products and IL-1beta. Cytokines increase leptin expression in the adipose tissue, and leptin may contribute to, but is also not essential for, the anorectic effects of microbial products and cytokines. In addition, a direct action of cytokines and microbial products on the central nervous system (CNS) is presumably involved in the anorexia during infection. Cytokines can reach CNS receptors through circumventricular organs and through active or passive transport mechanisms or they can act through receptors on endothelial cells of the brain vasculature and stimulate the release of subsequent mediators such as eicosanoids. De novo CNS cytokine synthesis occurs in response to peripheral infections, but its role in the accompanying anorexia is still open to discussion. Central mediators of the anorexia during infection appear to be neurochemicals involved in the normal control of feeding, such as serotonin, dopamine, histamine, corticotropin releasing factor, neuropeptide Y, and alpha-melanocyte-stimulating hormone. Reciprocal, synergistic, and antagonistic interactions between various pleiotropic cytokines, and between cytokines and neurochemicals, form a complex network that mediates the anorexia during infection. Current knowledge on the mechanisms involved suggests some therapeutic options for treatment. Substances that block common key steps in cytokine synthesis or cytokine action, or inhibitors of eicosanoid synthesis, may hold more promise than attempts to antagonize specific cytokines. To target the neurochemical mediation of the anorexia during infection may be even more efficient. Future research should address these neurochemical mechanisms and the cytokine actions at the blood-brain barrier. Further unanswered questions concern the modulation of the anorexia during infection by gender and nutritional state.

Examining the effects of lipopolysaccharide and cholecystokinin on water ingestion: comparing intake and palatability

Lipopolysaccharide (LPS) and cholecystokinin (CCK) have been shown to have anorectic properties in a variety of species. The present study examined the effects of LPS and CCK, both alone and in combination, on two different aspects of water ingestion, water intake and palatability. On test days, animals were first injected intraperitoneally (i.p.) with either LPS (200 microg/kg) or NaCl vehicle, and 2 h later received a second injection of either CCK (8 microg/kg) or NaCl vehicle. In Experiment 1, water intake was monitored for 1 h on 3 separate test days 72 h apart; while in Experiment 2, water palatability was assessed using the taste reactivity test (TRT), on two separate test days 72 h apart. Both LPS and CCK significantly (p<0.05) reduced water intake, with the effects of combined LPS with CCK being more pronounced than either agent injected alone. Rats developed a rapid tolerance to the effects of LPS on water intake on subsequent exposures to LPS. Results from the TRT indicated that LPS enhanced water palatability (p<0.05), as evidenced by a high level of ingestive responding, whereas CCK produced a pattern of responding indicative of satiety. LPS plus CCK reduced ingestive responding on the first test day, but these responses were significantly increased on the second test day (p<0.05). These results demonstrate that although LPS reduces water intake, it enhances water palatability. The results further underscore the necessity for examining palatability changes in addition to intake measures when studying the regulation of feeding and drinking.

T-lymphocyte activation increases hypothalamic and amygdaloid expression of CRH mRNA and emotional reactivity to novelty

Stimulation of T-cells with staphylococcal enterotoxin B (SEB) significantly elevates interleukin-2 (IL-2) and contemporaneous activation of the hypothalamic-pituitary-adrenal (HPA) axis and c-fos in the paraventricular nucleus (PVN) of BALB/cByJ mice. Such neural signaling may promote cognitive and emotional adaptation before or during infectious illness. Because corticotropin-releasing hormone (CRH) is an anxiogenic neuropeptide that may mediate the stressor-like effects of immunological stimuli, we measured neuronal CRH mRNA alterations in mice challenged with SEB. Increased CRH mRNA levels were observed in the PVN and central nucleus of the amygdala (ceA) 4-6 hr after SEB administration. This was associated with plasma ACTH increases, which could be abrogated by the systemic administration of anti-CRH antiserum. Additional experiments did not support a role for IL-2 or prostaglandin synthesis in activating the HPA axis. Behavioral experiments testing for conditioned taste aversion did not confirm that SEB challenge promotes malaise. However, consistent with the notion that central CRH alterations induced by SEB may affect emotionality (e.g., fear), SEB challenge augmented appetitive neophobia in a context-dependent manner, being marked in a novel and stressful environment. It is hypothesized that immunological stimuli generate a cascade of events that solicit integrative neural processes involved in emotional behavior. As such, these data support the contention that affective illness may be influenced by immunological processes and the production of cytokines and are consistent with other evidence demonstrating that autoimmune reactivity is associated with enhanced emotionality.

Physiological Mechanisms Causing Sickness Behaviour and Suffering in Diseased Animals

Disease is one of the most important causes of animal suffering. When diseases are treated the aim is to achieve rapid and permanent recovery and this helps to reduce the duration of suffering. It does not, however, alleviate suffering during the fulminant and recovery phases. Greater attention needs to be given to alleviating suffering and the signs of sickness during disease states. In this paper, the role of the cytokines in mediating sickness behaviour and suffering during disease is reviewed. The importance of sickness behaviour in improving the chances of recovery are considered, along with the potential use of anti-cytokine strategies in alleviating suffering in disease states.

Infection-induced anorexia: active host defence strategy

In association with fever production, decreased food consumption is the most common sign of infection. This effect is often regarded as an undesirable manifestation of sickness. However, evidence suggests that just as many behaviours have now been shown to modify immunocompetence, infection-induced anorexia is a behaviour systematically organised for pathogen elimination. That is, anorexia is an active defence mechanism that is beneficial for host defence. This review details the mechanism of infection-induced anorexia, placing it within the framework of the intricately organised acute phase response--the host response to infection. Furthermore, the evolutionary, behavioural, metabolic and immunological consequences of infection-induced anorexia are outlined, each providing evidence for the beneficial nature of this response. The evidence suggests that food restriction is one of the important behavioural strategies that organisms have evolved for the fight against pathogenic invasion. Nevertheless, such benefits require fine homeostatic control, as chronic undernutrition has deleterious consequences for host defence.

Bacterial products and the control of ingestive behavior: clinical implications

Bacterial products such as lipopolysaccharides (LPS) and muramyl peptides are delivered in the course of infections. They trigger the host's acute phase responses to bacterial infections and are probably involved in the accompanying hypophagia because LPS and muramyl dipeptide (MDP, the minimal immunologically active muramyl peptide) reduce food intake after parenteral administration in animals. LPS and MDP inhibit feeding synergistically through separate but interacting mechanisms. The hypophagic effects of LPS and MDP are presumably mediated by the combined actions of interleukin-1, tumor necrosis factor, and other cytokines. More work is required to understand the interactions between these cytokines, and between bacterial products and cytokines, before cytokine antagonists can be used for treatment of the hypophagia during bacterial infections. As the hypophagia seems to be an early mechanism of host defense, a treatment should be carefully considered. If an intervention is indicated because of a patient's poor condition, inhibitors of eicosanoid synthesis and glucocorticoids may hold more promise for therapy because such substances block LPS and MDP hypophagia. Although LPS can reduce food intake by direct action on the brain, presently available evidence indicates that systemic LPS acts primarily in the periphery to generate a neural signal that is transmitted to the brain and inhibits feeding through the vagus. The exact site where LPS acts on peripheral nerves remains to be identified. LPS hypophagia is conditionable, but conditioning cannot solely account for LPS hypophagia under most test conditions. Whether MDP hypophagia is also conditionable and mediated by vagal afferents is not yet known. All in all, the putative mediators and mechanisms of LPS and MDP hypophagia suggest some options for a treatment of the hypophagia during bacterial infection, but present knowledge about the mechanisms and interactions of the involved substances is still fragmentary and requires further investigation.

Paradoxical conditioning of the plasma copper and corticosterone responses to bacterial endotoxin

The cascade of physiologic mechanisms in response to infection, the acute phase response, is recognized as having a major role in host defense. Two such responses are an increase in plasma copper and activation of the hypothalamic-pituitary-adrenal axis, which are consistently reported to occur during bacterial infection. We aimed to determine whether the alterations in plasma copper and corticosterone were conditionable using the conditioned taste aversion paradigm. The regime involved the pairing of a novel-tasting saccharine solution (the conditioned stimulus) with lipopolysaccharide (the unconditioned stimulus). Seven days after the initial pairing of these stimuli (the test day), the saccharine solution was represented. Animals exposed to this condition displayed a significant decrease in plasma copper levels. In addition, these rats experienced a reduction in plasma corticosterone that was time dependent. Paradoxically, the conditioned response of both these variables were in a direction contrary to that reported during bacterial infection. These results suggest that some acute phase responses may condition as a rebound response, or in an opposing trend to that occurring as the initial reaction.

Behaviorally conditioned anorexia: role of gastric emptying and prostaglandins

The reduction of food intake in response to bacteria is posited to be a favourable host reaction. This report attempted to examine whether gastric emptying is involved in the known conditionability of this response. Additionally, this study investigated the role of prostaglandins in the conditioned anorexic response. To investigate this phenomenon, lipopolysaccharide (LPS) (100 micrograms/kg) was used as the unconditioned stimulus, and paired with a novel 1% saccharin solution (conditioned stimulus). Upon conditioned stimulus (CS) representation, experimental animals displayed a marked reduction in food consumption (experiment 1) and emptying of gastric contents (experiment 2). Additionally, treatment with indomethacin upon CS reexposition blocked both the conditioned anorexia and suppression of gastric emptying. These results indicate that conditioned anorexia is possibly the result of a conditioned inhibition of gastric emptying, and this process is mediated by conditioned alterations in PG levels.

Modification of body temperature and sleep state using behavioral conditioning

Previous research has demonstrated the conditionability of events within the acute-phase response. This study examined whether two such responses, fever and sleep alterations, were conditionable in the rat during the dark photoperiod. The experimental animals were administered a novel saccharin solution as the conditioned stimulus (CS) in conjunction with lipopolysaccharide as the unconditioned stimulus (UCS). This group displayed significantly higher body temperatures than controls upon saccharin representation, 7 days after the original CS-UCS pairing. The experimental animals additionally displayed a conditioned increase in slow wave sleep (SWS); however, the LPS-induced reduction in rapid eye movement (REM) sleep was unable to be reenlisted. Similar to the acute-response, the conditioned alteration in SWS appeared to be due to an increase in episode frequency, rather than duration. These results suggest that the multiple acute-phase events may be simultaneously conditionable, producing an optimum environment for pathogen elimination.