Central injection of interleukin-13 potentiates LPS-induced sickness behavior in rats.

Systemic administration of the bacterial endotoxin lipopolysaccharide (LPS) has profound depressive effects on behavior that are mediated by the inducible expression of proinflammatory cytokines such as interleukin-1 (IL-1), IL-6 and tumor necrosis factor-alpha (TNF-alpha) in the brain. To assess the regulatory effects of the anti-inflammatory cytokine IL-13 on LPS-induced sickness behavior, rats injected i.p. with LPS were administered rat recombinant IL-13 i.c.v. IL-13 (300 ng) potentiated the behavioral effects of LPS (125 microg/kg) when both molecules were co-injected. Administration of IL-13 (300 ng) 12 h prior to LPS (150 microg/kg) did not block the depressing effects of LPS on social exploration. These results indicate that IL-13 acts as a proinflammatory cytokine in the brain.

Effects of serotonin synthesis blockade on interleukin-1 beta action in the brain of rats.

The ability of the brain serotonergic system to mediate the effects of interleukin-1beta (IL-1beta) was investigated. Intracerebroventricular administration of IL-1beta induced a significant pyrogenic reaction, depression in social behaviour, loss of body weight and reduced food intake in rats. Pre-treatment with p-chlorphenylalanine, an inhibitor of serotonin synthesis, blocked the IL-1beta-induced decrease in food intake and loss of body weight, but failed to alter the temperature increase and the decrease in communicative activity.

Tumor necrosis factor(alpha) and insulin-like growth factor-I in the brain: is the whole greater than the sum of its parts?

The cytokine tumor necrosis factor(alpha) (TNFalpha) and the hormone insulin-like growth factor-I (IGF-I) have both been shown to regulate inflammatory events in the central nervous system (CNS). This review summarizes the seemingly independent roles of TNFalpha and IGF-I in promoting and inhibiting neurodegenerative diseases. We then offer evidence that the combined effects of IGF-I and TNFalpha on neuronal survival can be vastly different when both receptors are stimulated simultaneously, as is likely to occur in vivo. We propose the framework of a molecular model of hormone-cytokine receptor cross talk in which disparate cell surface receptors share intracellular substrates that regulate neuronal survival.

Chronic treatment with the atypical antidepressant tianeptine attenuates sickness behavior induced by peripheral but not central lipopolysaccharide and interleukin-1beta in the rat.

RATIONALE: The hypothesis that proinflammatory cytokines play a causative role in the pathophysiology of depression has been recently tested by studying the effect of antidepressants on production of endogenous cytokines, and on sickness behavior induced by exogenous cytokines. In this last case, however, the effect of antidepressants has been only studied on the effect of peripherally administered cytokines. OBJECTIVES: The aim of the present study was to determine whether the antidepressant tianeptine can attenuate both peripheral and central cytokine actions. METHODS: Rats were injected IP with acute (10 mg/kg) or chronic (10 mg/kg, 2 times/day, 17 days) tianeptine. The effects of this treatment were assessed on the behavioral (social exploration, locomotion) and metabolic (food intake, body weight) alterations induced by peripheral or central administration of the cytokine inducer lipopolysaccharide (LPS) (250 microg/kg IP; 100 ng/rat ICV) or the prototypical proinflammatory cytokine interleukin-1 (IL-1)beta (15 microg/rat IP; 90 ng/rat ICV). RESULTS: Chronic, but not acute, treatment with tianeptine attenuated the behavioral signs of sickness behavior induced by peripheral, but not central, LPS or IL-1beta. CONCLUSIONS: This work, which is the first in vivo study assessing the effect of an antidepressant on centrally induced immune activation, shows a clear dissociation between peripheral and central cytokine effects, and suggests a peripheral site of action of tianeptine. It also provides the first evidence that the protective effects of classical antidepressants on LPS-induced sickness behavior extend to an atypical antidepressant, and that the protective effect of antidepressants also applies to IL-1beta.

The vagus nerve mediates behavioural depression, but not fever, in response to peripheral immune signals; a functional anatomical analysis.

Cytokines act on the brain to induce fever and behavioural depression after infection. Although several mechanisms of cytokine-to-brain communication have been proposed, their physiological significance is unclear. We propose that behavioural depression is mediated by the vagus nerve activating limbic structures, while fever would primarily be due to humoral mechanisms affecting the preoptic area, including interleukin-6 (IL-6) action on the organum vasculosum of the laminae terminalis (OVLT) and induction of prostaglandins. This study assessed the effects of subdiaphragmatic vagotomy in rats on fever, behavioural depression, as measured by the social interaction test, and Fos expression in the brain. These responses were compared with induction of the prostaglandin-producing enzyme cyclooxygenase-2 and the transcription factor Stat3 that translocates after binding of IL-6. Vagotomy blocked behavioural depression after intraperitoneal injection of recombinant rat IL-1beta (25 microg/kg) or lipopolysaccharide (250 microg/kg; LPS) and prevented Fos expression in limbic structures and ventromedial preoptic area, but not in the OVLT. Fever was not affected by vagotomy, but associated with translocation of Stat3 in the OVLT and cyclooxygenase-2 induction around blood vessels. These results indicate that the recently proposed vagal link between the immune system and the brain activates limbic structures to induce behavioural depression after abdominal inflammation. Although the vagus might play a role in fever in response to low doses of LPS by activating the ventromedial preoptic area, it is likely to be overridden during more severe infection by action of circulating IL-6 on the OVLT or prostaglandins induced along blood vessels of the preoptic area.

Role of interleukin-1beta and tumour necrosis factor-alpha in lipopolysaccharide-induced sickness behaviour: a study with interleukin-1 type I receptor-deficient mice.

Interleukin-1 (IL-1) mediates symptoms of sickness during the host response to infection. IL-1 exerts its effects via several subtypes of receptors. To assess the role of IL-1 receptor type I (IL-1RI) in the sickness-inducing effects of IL-1, IL-1beta and the cytokine inducer lipopolysaccharide were administered to IL-1RI-deficient mice (IL-1RI-/-). Sickness was assessed by depression of social exploration, anorexia, immobility and body weight loss. IL-1RI-/- mice were resistant to the sickness-inducing effects of IL-1beta administered intraperitoneally (2 microg/mouse) and intracerebroventricularly (2 ng/mouse), but still fully responsive to lipopolysaccharide administered intraperitoneally (2.5 microg/mouse) and intracerebroventricularly (3 ng/mouse). The sensitivity of IL-1RI-/- mice to lipopolysaccharide was not due to a higher brain expression of proinflammatory cytokines other than IL-1, since lipopolysaccharide-induced expression of brain IL-1 beta, tumour necrosis factor-alpha (TNF-alpha) and IL-6 transcripts were identical in IL-1RI-/- and control mice when measured by semiquantitative reverse-transcriptase polymerase chain reaction 1 h after treatment. Blockade of TNF-alpha action in the brain by intracerebroventricular administration of a fragment of the soluble TNF receptor, TNF binding protein (3.6 microg/mouse), attenuated the depressive effects of intraperitoneal injection of lipopolysaccharide (1 microg/mouse) on behaviour in IL-1RI-/- but not in control mice. Since IL-1RI-/- mice were not more sensitive to intracerebroventricularly TNF-alpha (50 ng) than control mice, these results indicate that IL-1RI mediates the sickness effect of IL-1 and that TNF-alpha simply replaces IL-1 when this last cytokine is deficient.

Role of IL-6 in cytokine-induced sickness behavior: a study with IL-6 deficient mice.

Interleukin-6 (IL-6) is synthesized and released in response to the cytokine inducer lipopolysaccharide (LPS) and IL-1, and acts as an endogenous pyrogen. Systemic administration of LPS and IL-1 to mice induces signs of sickness, including reduction of social exploration, immobility and body weight loss. To assess the role of IL-6 in the induction of sickness behavior, male IL-6-deficient mice (IL-6 -/-, Balb/cAn genetic background) were used and compared to IL-6 +/+ littermates. The depressing effects of intraperitoneal LPS (2.5 microg/mouse) and IL-1beta (1.0 microg/mouse) on behavior and change in body weight were more marked in IL-6 +/+ than in IL-6 -/- mice. The same difference was observed when mice were injected with LPS (5 ng/mouse) and IL-1beta (1 ng/mouse) into the lateral ventricle of the brain (i.c.v.). These results show that IL-6 released at the periphery and /or in the central nervous system plays a role in the behavioral response to LPS and IL-1.

Vagotomy attenuates the behavioural but not the pyrogenic effects of interleukin-1 in rats.

Vagal afferent signals, have been implicated in cytokine mediated interactions between the periphery and the central nervous system. Studies in experimental animals have shown that cytokine induced activation of brain mediated responses to infection such as fever, sickness behaviour and pituitary-adrenal activation, are inhibited by subdiaphragmatic vagotomy. We have previously proposed that the peripheral signal to the brain in fever is of a humoral nature while others have suggested that either neural afferents or a mixture of both humoral and neural signals may be involved. The objective of the present study was to examine further the role of vagal transmission, in mediating the febrile response to a systemic injection of IL-1beta in rats and to compare this with changes in social exploration behaviour. Intraperitoneal injection of IL-1beta (1.0-30.0 microg/kg) inhibited social exploration in rats and this was attenuated in vagotomized animals. Injection of increasing concentrations of IL-1beta (0.1-1.0 microg/rat) induced significant (P<0.001) increases in core body temperature. However, in contrast to effects on social exploration, the increase in temperature was not inhibited by vagotomy at any of the doses used. These observations demonstrate a dissociation between the two brain mediated events, one of which is dependent on the integrity of the vagus nerve (social exploration) while the other (fever) is apparently generated by different mechanisms which may include circulating pyrogens.

Neural and humoral pathways of communication from the immune system to the brain: parallel or convergent?

The first studies carried out on the mechanisms by which peripheral immune stimuli signal the brain to induce fever, activation of the hypothalamic-pituitary-adrenal axis and sickness behavior emphasized the importance of fenestrated parts of the blood-brain barrier known as circumventricular organs for allowing blood-borne proinflammatory cytokines to act on brain functions. The discovery in the mid-1990s that subdiaphragmatic section of the vagus nerves attenuates the brain effects of systemic cytokines, together with the demonstration of an inducible brain cytokine compartment shifted the attention from circumventricular organs to neural pathways in the transmission of the immune message to the brain. Since then, neuroanatomical studies have confirmed the existence of a fast route of communication from the immune system to the brain via the vagus nerves. This neural pathway is complemented by a humoral pathway that involves cytokines produced at the level of the circumventricular organs and the choroid plexus and at the origin of a second wave of cytokines produced in the brain parenchyma. Depending on their source, these locally produced cytokines can either activate neurons that project to specific brain areas or diffuse by volume transmission into the brain parenchyma to reach their targets. Activation of neurons by cytokines can be direct or indirect, via prostaglandins. The way the neural pathway of transmission interacts with the humoral pathway remains to be elucidated.

Expression and localization of p80 and p68 interleukin-1 receptor proteins in the brain of adult mice.

The biological effects of interleukin-1 (IL-1) are mediated by two distinct receptors, the p80 type I IL-1 and p68 type II IL-1 receptor proteins (IL-1RI and IL-1RII, respectively), both of which have been recently co-localized to the growth hormone synthesizing cells of the adenohypophysis. Previous studies have shown that IL-1 can bind to specific structures in the central nervous system, but the distribution of IL-1RI and IL-1RII proteins in the adult mouse brain has not been reported. Here we have used immunohistochemistry to study the expression, distribution and cellular localization of both isoforms of the IL-1 receptor proteins in the adult mouse brain. Using a combination of processing techniques (AMeX fixation and cryosectioning), we have immunolabeled brain sections for each isoform of the IL-1R. Both isoforms are expressed in the CNS, particularly in neuronal soma of the granular layer of the dentate gyrus and pyramidal cells of fields CA1-CA4 of Ammon's horn of the hippocampus, in epithelial cells of the choroid plexus and ependymal layer, and in neuronal soma of Purkinje cells of the cerebellum. The IL-1RII isoform, but not IL-1RI, is expressed in specific neuronal soma and proximal cell processes of neurons of the paraventricular gray matter of the hypothalamus. These immunohistochemical data directly demonstrate the neuronal expression of both IL-1R proteins in situ. The distribution and cellular localization of IL-1R proteins in the CNS provide a molecular basis for understanding reciprocal interactions between the immune system and the brain.

Central injection of IL-10 antagonizes the behavioural effects of lipopolysaccharide in rats.

Peripheral (i.p.) and central (i.c.v.) injections of lipopolysaccharide (LPS) have been shown to induce brain expression of proinflammatory cytokines and to depress social behaviour in rats, increase duration of immobility and induce body weight loss. To determine if the anti-inflammatory cytokine, interleukin-10 (IL-10) is able to modulate these effects, recombinant rat IL-10 was injected in the lateral ventricle of the brain (30, 100, 300 ng/rat) prior to i.p. or i.c.v. injection of LPS (250 micrograms/kg or 60 ng/rat, respectively). Social exploration was depressed for 6 h after i.p. LPS injection. This effect was attenuated by IL-10 (30 and 100 ng) 2 h after injection, whereas the highest dose of IL-10 blocked the depression of social interaction for 6 h after LPS injection. IL-10 produced the same effects on the increase of immobility although the results did not reach significance. Social exploration was depressed 3 h after i.c.v. LPS injection, and this was accompanied by increased immobility. These effects were totally blocked by i.c.v. IL-10 (300 ng/rat). Rats lost body weight after i.c.v. LPS, and this effect was attenuated by i.c.v. IL-10. These results indicate that IL-10 is able to modulate the production and/or action of central proinflammatory cytokines.

Central administration of rat IL-6 induces HPA activation and fever but not sickness behavior in rats.

Interleukin (IL)-6 has been proposed to mediate several sickness responses, including brain-mediated neuroendocrine, temperature, and behavioral changes. However, the exact mechanisms and sites of action of IL-6 are still poorly understood. In the present study, we describe the effects of central administration of species-homologous recombinant rat IL-6 (rrIL-6) on the induction of hypothalamic-pituitary-adrenal (HPA) activity, fever, social investigatory behavior, and immobility. After intracerebroventricular administration of rrIL-6 (50 or 100 ng/rat), rats demonstrated HPA and febrile responses. In contrast, rrIL-6 alone did not induce changes in social investigatory and locomotor behavior at doses of up to 400 ng/rat. Coadministration of rrIL-6 (100 ng/rat) and rrIL-1beta (40 ng/rat), which alone did not affect the behavioral responses, reduced social investigatory behavior and increased the duration of immobility. Compared with rhIL-6, intracerebroventricular administration of rrIL-6 (100 ng/rat) induced higher HPA responses and early-phase febrile responses. This is consistent with a higher potency of rrIL-6, compared with rhIL-6, in the murine B9 bioassay. We conclude that species-homologous rrIL-6 alone can act in the brain to induce HPA and febrile responses, whereas it only reduces social investigatory behavior and locomotor activity in the presence of IL-1beta.

Biological activity and brain actions of recombinant rat interleukin-1alpha and interleukin-1beta.

IL-1alpha and IL-1beta have potent effects on the central nervous system resulting in fever, activation of the hypothalamic-pituitary-adrenal axis and behavioural depression. These effects have mainly been studied in rats, using recombinant human and mouse IL-1. Because IL-1alpha and IL-1beta show some species specificity in the potency of their biological activities, the objective of the present work was to directly compare the effects of recombinant rat IL-1alpha and IL-1beta in the rat system as a first step to dissect out the mechanisms that are involved in these effects. In vitro, recombinant rat IL-1alpha and IL-1beta bound with the same affinity as human IL-1 to the rat insulinoma Rin m5F cell line that mainly expresses type I IL-1 receptors. This binding activated IL-1 receptors, as shown by induction of the synthesis of TNF-alpha mRNA. In vivo, recombinant rat IL-1alpha and IL-1beta enhanced body temperature, increased plasma levels of corticosterone and ACTH, and depressed social behaviour. All these effects were obtained at doses 100-1,000 fold lower when IL-1 was injected centrally than when it was administered peripherally, indicating that they are centrally mediated. The relative potencies of recombinant rat IL-1alpha and IL-1beta were not the same depending on the endpoint and the route of injection, indicating that different mechanisms are likely to be involved in the various effects of IL-1 on the brain.

Cytokines and sickness behavior.

Peripheral and central injections of interleukin-1 (IL-1) and lipopolysaccharide (LPS) induce the expression of proinflammatory cytokines in the brain and have profound depressing effects on spontaneous and learned behaviors. These effects are mediated by vagal afferents, because they are abrogated by section of the vagus nerves at the subdiaphragmatic level in rats and mice. Vagotomy does not interfere with the synthesis and release of proinflammatory cytokines at the periphery, because plasma and tissue levels of interleukin-1 of vagotomized animals are similar to those of sham-operated animals. Furthermore, the consequences of vagotomy on the host behavioral response to peripheral cytokines are specific to the intraperitoneal route of administration of cytokines because vagotomized animals are still able to respond to IL-1 injected intravenously, subcutaneously, and into the lateral ventricle of the brain. Finally, substance P and cholecystokinin do not appear to play a key role in the transmission of the immune message to the brain because pretreatment by capsaicin or by specific antagonists of CCKA and CCKB receptors does not alter the behavioral effects of LPS and IL-1. All these findings point to the role of neural afferents for transmitting the immune message from the periphery to the brain.

Interleukin-1-induced sickness behavior depends on behavioral lateralization in mice.

Inter-individual differences in brain-immune interactions have been demonstrated previously in mice using lateralization as a behavioral trait of population heterogeneity. Lipopolysaccharide (LPS), which is known to induce neurochemical, neuroendocrine, and immune responses depending on lateralization, is also able to induce sickness behavior, via the production of interleukin-1 (IL-1). The objective of this study was to determine whether lateralization can influence the behavioral response to LPS and to IL-1. To test this hypothesis, adult female C3H mice, previously selected for paw preference in a food reaching task, were injected intraperitoneally (i.p.) with 0.75 microg LPS or 0.75 microg recombinant IL-1beta. Sickness induced by these molecules was measured by depressed social behavior, increased immobility, loss of body weight, and reduced food intake during the 6 h following injection. LPS-induced sickness was similar in right- and left-pawed mice. In contrast, IL-1-induced sickness behavior was dependent on behavioral lateralization. IL-1-induced depression of social investigation was more pronounced in right-pawed mice than in left-pawed animals. Likewise, IL-1-induced immobility was more important in right-pawed mice. There was a similar trend for food intake to be lower and loss of body weight to be higher in right-pawed mice than in left-pawed animals. These results demonstrate that right-pawed mice are more sensitive to IL-1-induced sickness than left-pawed animals. They extend our previous data showing a greater susceptibility to stress of right-pawed animals. The existence of inter-individual differences in the reactivity to stress or immune activation may be useful to study the mechanisms of the various strategies used by an individual in response to environmental aggressions.

Cholecystokinin receptors do not mediate the behavioral effects of lipopolysaccharide in mice.

To test the possible role of cholecystokinin (CCK) in the decrease of social exploration induced by intraperitoneal (IP) injection of lipopolysaccharide (LPS, 100 microg/kg), mice were pretreated with IP or intracerebroventricular (ICV) injection of the CCKA receptor antagonist L-364,718 (3 mg/kg and 10 microg/kg, respectively) and the CCKB receptor antagonist L-365,260 (1 mg/kg and 10 microg/kg, respectively). L-364,718 and L-365,260 did not alter LPS-induced decrease in social investigation, whatever the route of administration, suggesting that endogenous cholecystokinin does not mediate the effect of proinflammatory cytokines on social exploration in mice.

Central interleukin-1 receptors as mediators of sickness.

These data establish that cytokines, such as IL-1, can act on specific receptors within the brain to induce many symptoms of sickness. A number of inflammatory stimuli in the periphery can activate both the transcription and translation of IL-1 within the central nervous system. It will now be important to determine if similar central IL-1 pathways are activated during SLE and whether these central inflammatory cytokines are involved in the neurologic complications that often accompany this disease.

Hypersensitivity of lurcher mutant mice to the depressing effects of lipopolysaccharide and interleukin-1 on behaviour.

Lurcher mutant mice are characterized by a fast and almost total loss of olivocerebellar neurones during the first postnatal month, associated with a chronic inflammatory state. To test their brain sensitivity to proinflammatory cytokines, we assessed the behavioural responses of adult male Lurcher and wild type to an i.p. or i.c.v. injection of rat recombinant IL-1 beta, and lipopolysaccharide (LPS). IL-1 beta (15 micrograms kg-1, i.p. or 1 ng i.c.v.) decreased social exploration measured 2, 4 and 6 h later, and this decrease was significantly more pronounced in Lurcher than in wild type mice. LPS (60 micrograms kg-1, i.p. or 5 ng i.c.v.) decreased social exploration measured 2 and 4 h later, and this effect was also significantly more marked in Lurcher than in wild type mice. These results suggest that the chronic inflammatory state which characterizes Lurcher mice renders these animals more sensitive to the effects of cytokines such as IL-1 beta and LPS. This difference may be due to the higher reactivity of brain macrophages and glial cells to LPS and IL-1 in Lurcher mice than in wild type.