Role of brain transmigrating neutrophils in depression-like behavior during systemic infection.

Peripheral inflammation induces transmigration of interleukin (IL)-1ß-expressing neutrophils to the brain. We investigated the possibility that this presents a new route of immune-to-brain communication by assessing their role in sickness behaviors relevant for mood disorders. Mice treated with lipopolysaccharide (LPS) developed despair-like behavior, and administration of an anti-polymorphonuclear antibody abolished LPS-induced despair-like and asocial behaviors, which correlated with the levels of IL-1ß expression in the brain. These behavioral changes were directly mediated by the energy-regulating hormone, leptin. Increasing the concentration of endogenous leptin during obesity exacerbated, whereas its neutralization using a specific antiserum attenuated sickness behaviors and importantly the neutrophil transmigrating process. Our results indicate a role for peripheral neutrophils in conveying inflammatory signals to the brain, which appears to be dependent on the energy status of the organism. This constitutes a novel mechanism of immune-to-brain communication relevant to mood disorders.

Effect of obesity on the acute inflammatory response in pregnant and cycling female rats.

Nonpregnant female rats have a lower inflammatory response to lipopolysaccharide (LPS) than males and, at late stages of gestation, the fever response to this immunogen is almost completely suppressed. We have shown in males that obesity exacerbates sickness responses to pathogenic stimuli. In the present study, we investigated whether obesity would have a similar effect in females and reverse some of the suppressive effects of pregnancy on the innate immune response. Lean and diet-induced obese adult Wistar rats were randomly separated into either cycling or mated groups. On day 18 of pregnancy or in the metestrous/dioestrous phase in cycling rats, a single injection of LPS (100 µg/kg) was administered and rats were sacrificed 8h or 24 h later. In pregnant females, LPS induced a higher increase in body temperature in obese rats only at the 24-h time point and lower hypothalamic interleukin (IL)-1ß expression and higher circulating levels of IL-1 receptor antagonist (ra) than their cycling counterparts. Conversely, there was no suppression of inflammatory signals in the white adipose tissue of pregnant rats. At 24 h post LPS, the cell surface marker CD11c and IL-6 mRNA expression were increased in white adipose tissue from obese rats regardless of reproductive state, whereas IL-1ra was highest in the LPS-treated obese pregnant group. In cycling females, LPS induced a higher fever response in obese rats accompanied by higher circulating levels of IL-6 and IL-1ra, as well as an increase in circulating leptin only in the obese cycling group. In the hypothalamus, obese rats showed significantly higher expression of nuclear factor-IL-6 in at the 8-h time point. Collectively, these results show that diet-induced obesity in females is associated with a similar pattern of response to that previously observed in males. On the other hand, obesity had limited effects in pregnant rats, with the exception of white adipose tissue.

Leptin regulates leukocyte recruitment into the brain following systemic LPS-induced inflammation.

The appetite suppressing hormone leptin has emerged as an important modulator of immune function and is now considered to be a critical link between energy balance and host defense responses to pathogens. These 'adaptive' responses can, in situations of severe and sustained systemic inflammation, lead to adverse effects including brain damage that is partly mediated by neutrophil recruitment into the brain. We examined the contribution of leptin to this process in leptin-deficient (ob/ob), -resistant (db/db) and wild-type (WT) mice injected intraperitoneally with a septic dose of lipopolysaccharide (LPS). This treatment induced a dramatic increase in the number of neutrophils entering the brain of WT mice, an effect that was almost totally abolished in the mutant mice and correlated with a significant reduction in the mRNA levels of interleukin-1beta, intracellular adhesion molecule-1 and neutrophil-specific chemokines. These effects were reversed with leptin replenishment in ob/ob mice leading to recovery of neutrophil recruitment into the brain. Moreover, 48 h food deprivation in WT mice, which decreased circulating leptin levels, attenuated the LPS-induced neutrophil recruitment as did a single injection of an anti-leptin antiserum 4 h before LPS treatment in WT mice. These results provide the first demonstration that leptin has a critical role in leukocyte recruitment to the brain following severe systemic inflammation with possible implications for individuals with altered leptin levels such as during obesity or starvation.

Central nervous action of interleukin-1 mediates activation of limbic structures and behavioural depression in response to peripheral administration of bacterial lipopolysaccharide.

Although receptors for the pro-inflammatory cytokine interleukin-1 have long been known to be expressed in the brain, their role in fever and behavioural depression observed during the acute phase response (APR) to tissue infection remains unclear. This may in part be due to the fact that interleukin-1 in the brain is bioactive only several hours after peripheral administration of bacterial lipopolysaccharide (LPS). To study the role of cerebral interleukin-1 action in temperature and behavioural changes, and activation of brain structures during the APR, interleukin-1 receptor antagonist (IL-1ra; 100 microg) was infused into the lateral brain ventricle 4 h after intraperitoneal (i.p.) LPS injection (250 microg/kg) in rats. I.p. LPS administration induced interleukin-1beta (IL-1beta) production in systemic circulation as well as in brain circumventricular organs and the choroid plexus. Intracerebroventricular (i.c.v.) infusion of IL-1ra 4 h after i.p. LPS injection attenuated the reduction in social interaction, a cardinal sign of behavioural depression during sickness, and c-Fos expression in the amygdala and bed nucleus of the stria terminalis. However, LPS-induced fever, rises in plasma corticosterone, body weight loss and c-Fos expression in the hypothalamus and caudal brainstem were not altered by i.c.v. infusion of IL-1ra. These findings, together with our previous observations showing that i.c.v. infused IL-1ra diffuses throughout perivascular spaces, where macrophages express interleukin-1 receptors, can be interpreted to suggest that circulating or locally produced brain IL-1beta acts on these cells to bring about behavioural depression and activation of limbic structures during the APR after peripheral LPS administration.

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

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

Interleukin-1 receptor antagonist as a modulator of gender differences in the febrile response to lipopolysaccharide in rats.

Febrile responses to bacterial pathogens are attenuated near term of pregnancy in several mammalian species. It is unknown, however, whether this reflects a fundamental physiological adaptation of female rats or whether it is specific to pregnancy. The aims of this study therefore were 1) to determine whether febrile responses to the bacterial endotoxin lipopolysaccharide (LPS) are attenuated in female vs. male rats and, if so, to identify possible mechanisms involved in modulating this and 2) to assess whether plasma concentrations of the anti-inflammatory cytokine, interleukin-1 receptor antagonist (IL-1ra), an important regulator of fever, are dependent on the physiological state of the female and could therefore be involved in modulating febrile responses. We found febrile responses were attenuated in cycling female vs. male rats and also in near-term pregnant dams vs. cycling females after intraperitoneal injection of LPS (0.05 mg/kg). Plasma levels of IL-1ra were significantly greater in female rats after injection of LPS, particularly during pregnancy, than in males. This was accompanied by attenuated levels of hypothalamic IL-1beta and cyclooxygenase-2 mRNA, two key mediators of the febrile response, in female rats. Furthermore, increasing plasma levels of IL-1ra in male rats by intraperitoneal administration of the recombinant antagonist attenuated hypothalamic mRNA levels of these mediators after LPS. These data suggest that there is a fundamental difference in febrile response to LPS between the genders that is likely regulated by IL-1ra. This may be an important mechanism that protects the developing fetus from potentially deleterious consequences of maternal fever during pregnancy.

The role of cytokines in mediating effects of prenatal infection on the fetus: implications for schizophrenia.

Maternal infections with bacterial or viral agents during pregnancy are associated with an increased incidence of schizophrenia in the offspring at adulthood although little is known about the mechanism by which maternal infection might affect fetal neurodevelopment. Exposure of pregnant rodents to the bacterial endotoxin, lipopolysaccharide (LPS), results in behavioral deficits in the adult offspring that are relevant to schizophrenia. It is however unknown whether these effects are due to the direct action of the inflammatory stimulus on the developing fetus, or due to secondary immune mediators (cytokines) activated at maternal/fetal sites. In this study we sought to elucidate the site of action of LPS, following a single intraperitoneal (i.p.) injection, in pregnant rats at gestation day 18. Animals received 5 muCi of iodinated LPS ((125)I-LPS) and its distribution was assessed in maternal/fetal tissues (1-8 h). In addition, induction of the inflammatory cytokines, TNF-alpha, IL-1beta and IL-6, was measured in maternal/fetal tissues following maternal LPS challenge (0.05 mg/kg, i.p.) (2-8 h). (125)I-LPS was detected in maternal tissues and placenta, but not the fetus. This distribution was accompanied by significant increases in TNF-alpha, IL-1beta and IL-6 in maternal plasma and placenta, but not in fetal liver or brain. A significant increase in IL-1beta was however detected in fetal plasma, possibly due to transfer from the maternal circulation or placenta. Collectively, these data suggest that effects of maternal LPS exposure on the developing fetal brain are not mediated by the direct action of LPS, but via indirect actions at the level of the maternal circulation or placenta.

Role of endogenous interleukin-1 receptor antagonist in regulating fever induced by localised inflammation in the rat.

1. Interleukin (IL)-1 is a mediator of host defence responses to inflammation and injury, including fever, but its sites of synthesis and action have not been fully elucidated. The actions of IL-1 are antagonised by IL-1 receptor antagonist (IL-1ra). The present study tested the hypothesis that IL-1 and IL-1ra are produced locally at sites of peripheral inflammation in rats, and that endogenous IL-1ra acts to limit the fever resulting from the inflammation. 2. Injection of lipopolysaccharide (LPS; 100 microg kg-1) into a subcutaneous air pouch (I.PO.) of rats induced a significant increase in body temperature. Virtually all (approximately 85 %) of the injected LPS was recovered from the pouch between 1 and 8 h (when the experiment was terminated) after injection of LPS, but LPS was undetectable (< 50 pg ml-1) in plasma at any time. Concentrations of immunoreactive IL-1alpha and IL-1beta were increased significantly in the pouch at 1, 2, 3, 5 and 8 h after injection of LPS, corresponding with the rise in body temperature and the fever peak. The appearance of IL-1ra was delayed until 2 h. Thereafter, the concentrations of IL-1beta and IL-1ra increased in parallel with the development of fever, while the concentrations of IL-1alpha remained constant. IL-1ra, but not IL-1alpha or IL-1bet, was detected in significant quantities in the plasma of LPS-injected animals. 3. Treatment of rats with an anti-IL-1ra serum (2 ml, I.PO.) at the time of injection of LPS (10 or 100 microg kg-1, I.PO.) abolished the appearance of IL-1ra in the circulation. Although neutralisation of endogenous IL-1ra did not affect the maximum body temperature reached after injection of submaximum (10 microg kg-1, I.PO.) or maximum (100 microg kg-1, I.PO.) doses of LPS, the duration of the fever was significantly prolonged, and was associated with a 3- to 4-fold increase in immunoreactive IL-1beta concentrations in the pouch fluid, but not in the plasma, at the 8 h time point. 4. These data show that effects of local (I.PO.) injection of LPS are not due to its action in the circulation or at distant sites (such as at the blood-brain barrier). These data also show that locally produced IL-1ra, in response to injection (I.PO.) of LPS, inhibits the production and/or action of locally produced IL-1beta. The ability of IL-1ra to limit the duration, rather than the magnitude of the fever, is consistent with its delayed production, relative to IL-IL-1ra, therefore, appears to play a key role in the resolution of fever induced by localised inflammatory responses.

Interleukin (IL)-6 release and fever induced by a pre-formed pyrogenic factor (PFPF) derived from LPS-stimulated macrophages.

A novel pre-formed pyrogenic factor (PFPF), released by LPS-stimulated macrophages, has been identified, that induces an indomethacin-resistant fever. Its activity has to date not been found to match that of any described cytokine. In this study we observed that PFPF induced the release of large amounts of IL-6 from rat peritoneal macrophages. A combination of anti-cytokine antibodies and heat treatment excluded IL-1, tumor necrosis factor (TNF)-alpha and lipopolysaccharide (LPS) as being responsible for this effect. PFPF also induced interleukin (IL)-1, IL-6 and TNF-alpha in a subcutaneous air pouch, as well as increasing plasma IL-6, and induced a fever of 0.58 +/- 0.07 degrees C (3 hours) that was not reduced by indomethacin (2 mg/kg, ip). Preparative isoelectric focusing (IEF) showed that the material responsible for inducing IL-6 release had a pI between 4.7 and 5.8 and corresponded to the IEF pool that induced fever when injected intracerebroventricularly.

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.

Circulating interleukin-6 mediates the febrile response to localised inflammation in rats.

Interleukin (IL)-6 is an important mediator of the host response to disease and has been proposed, largely based upon circumstantial evidence, as the principal endogenous circulating pyrogen responsible for activating CNS mechanisms in fever during infection and inflammation. In the present investigation, we studied the role of peripheral IL-6 in fever and its relationship with IL-1, itself an important endogenous pyrogen and a potent stimulus of IL-6 production. Injection of lipopolysaccharide (LPS) into a sterile, subcutaneous air pouch (i.po.) in rats evoked an increase in body temperature which peaked at 3 h, and which was abolished in animals pretreated (intraperitoneally) with IL-6 antiserum. The increase in body temperature was accompanied by a significant elevation in concentrations of (immunoreactive) IL-1 and IL-6 at the site of inflammation (pouch), but only IL-6 in the circulation and cerebrospinal fluids. We propose that much of the circulating IL-6 originates at the site of inflammation, since injection of human recombinant (hr)IL-6 (i.po.) was detected (10 min after the injection) in the plasma using an ELISA specific for human IL-6. However, despite the relatively high concentration of IL-6 injected (25 microg kg-1, i.po.), this cytokine had no effect on body temperature when injected alone, but did induce fever when co-injected with a non-pyrogenic dose (when given alone) of IL-1beta, and exacerbated the fever to a pyrogenic dose of IL-1beta. The results from the present study demonstrate that IL-6 is a circulating endogenous pyrogen during LPS-induced fever, which acts in concert with IL-1beta at the local site of inflammation, before entering the circulation. Circulating IL-6 can then activate CNS mechanisms resulting in the development of the febrile response during disease.

IL-1beta induced changes in hypothalamic IL-1R1 and IL-1R2 mRNA expression in the rat.

Interleukin-1 receptor (IL-1R1 and IL-1R2) mRNA expression was detected within the rat hypothalamus, a primary site of IL-1 action, using RT-PCR. Levels of expression were unchanged by cardiac saline-perfusion. However, intracerebroventricular (i.c.v.) administration of IL-1beta caused changes in receptor mRNA expression in non-perfused animals that were profoundly different to those observed in their saline-perfused counterparts. This study demonstrates the importance of perfusing tissue to remove blood cells when determining changes in IL-1 receptor mRNA expression.

Detection of the interleukin 18 family in rat brain by RT-PCR.

Interleukin 18, an inflammatory cytokine, mediates its effects by interaction with its receptor complex, consisting of the IL-18 receptor (IL-18R) and receptor accessory protein (AcPL). A functional inhibitor of IL-18, the IL-18 binding protein (IL-18BP), has been identified recently. This study reports the detection of IL-18, IL-18R, AcPL and IL-18BP mRNA expression in the brain of normal adult rats using RT-PCR.

Cortical cell death induced by IL-1 is mediated via actions in the hypothalamus of the rat.

The cytokine IL-1 mediates diverse forms of neurodegeneration, but its mechanism of action is unknown. We have demonstrated previously that exogenous and endogenous IL-1 acts specifically in the rat striatum to dramatically enhance ischemic and excitotoxic brain damage and cause extensive cortical injury. Here we tested the hypothesis that this distant effect of IL-1 is mediated through polysynaptic striatal outputs to the cortex via the hypothalamus. We show that IL-1beta injected into the rat striatum with the excitotoxin alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (S-AMPA) caused increased expression of IL-1beta (mRNA and protein) mainly in the cortex where maximum injury occurs. Marked increases in IL-1beta mRNA and protein were also observed in the hypothalamus. S-AMPA, injected alone into the striatum, caused only localized damage, but administration of IL-1beta into either the striatum or the lateral hypothalamus immediately after striatal S-AMPA resulted in widespread cell loss throughout the ipsilateral cortex. Finally we showed that the cortical cell death produced by striatal coinjection of S-AMPA and IL-1beta was significantly reduced by administration of the IL-1 receptor antagonist into the lateral hypothalamus. These data suggest that IL-1beta can act in the hypothalamus to modify cell viability in the cortex. We conclude that IL-1-dependent pathways project from the striatum to the cortex via the hypothalamus and lead to cortical injury, and that these may contribute to a number of human neurological conditions including stroke and head trauma.

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.

Interleukin 1 in the brain: biology, pathology and therapeutic target.

The cytokine interleukin 1 (IL-1) has diverse actions in the brain. In normal brain the IL-1 system is expressed at low levels and is upregulated rapidly in response to local or peripheral insults. IL-1 mediates host defence responses to local and systemic disease and injury (e.g. fever, slow-wave sleep, appetite suppression and neuroendocrine responses) and to neuroinflammation and cell death in neurodegenerative conditions, such as stroke and head injury. It has also been implicated in chronic degenerative diseases, in particular, multiple sclerosis, Parkinson's and Alzheimer's diseases. The mechanisms regulating the expression and action of IL-1 are poorly understood, but involve multiple effects on neuronal, glial and endothelial cell function. Thus, the IL-1 system provides an attractive and intensely competitive target for therapeutic intervention.

Brain sites of action of endogenous interleukin-1 in the febrile response to localized inflammation in the rat.

1. Interleukin (IL)-1 is a potent endogenous pyrogen which causes fever when injected into a number of brain sites. However, the brain sites at which endogenous IL-1 acts to influence body temperature remain equivocal. The aim of this study was to determine the effect of local administration of the interleukin-1 receptor antagonist (IL-1ra) into specific sites in the hypothalamus, and other brain regions known to contain receptors for IL-1, on the febrile response of rats to peripheral injection of lipopolysaccharide (LPS) into a subcutaneous air pouch (intrapouch, i.p.o.) that does not lead to LPS appearance in the circulation. 2. Injection of LPS (100 microgram kg-1, i.p.o.) induced a rise in body temperature which commenced 1.5 h after injection and was maximal at 3 h (38.9 +/- 0.2 C, compared with 37.0 +/- 0.1 C at 0 h, n = 6, P < 0.001). Intracerebroventricular (i.c.v.) IL-1ra (500 microgram in 5 microliter) significantly attenuated LPS fever (IL-1ra, 37.7 +/- 0.2 C; saline, 38.9 +/- 0.2 C; n = 6, P < 0.001). Unilateral microinjection of IL-1ra (50 microgram in 0.5 microliter at 0 + 1 h) into the anterior hypothalamus (AH), paraventricular hypothalamic nucleus (PVH), peri-subfornical organ, subfornical organ (SFO) or hippocampus (dentate gyrus and CA3 region) also significantly reduced the fever induced by LPS. 3. The same dose of IL-1ra had no effect on fever when administered into the ventromedial hypothalamus (VMH), organum vasculosum lamina terminalis (OVLT), CA1 field of the hippocampus, striatum or cortex. 4. These data indicate that the action of endogenous IL-1 in the brain during fever is site specific, acting at the AH, PVH, SFO and hippocampus, but not the VMH, OVLT and striatum or cortex.

Leptin actions on food intake and body temperature are mediated by IL-1.

Leptin regulates energy balance through its actions in the brain on appetite and energy expenditure and also shares properties with cytokines such as IL-1. We report here that leptin, injected into rats intracerebroventricularly or peripherally, induces significant dose-dependent increases in core body temperature as well as suppression of appetite. Leptin failed to affect food intake or body temperature in obese (fa/fa) Zucker rats, which posses a defective leptin receptor. Furthermore, injection of leptin increased levels of the proinflammatory cytokine IL-1beta in the hypothalamus of normal Sprague-Dawley rats. Central injection of IL-1 receptor antagonist (IL-1ra) inhibited the suppression of food intake caused by central or peripheral injection of leptin (60 and 84%, respectively) and abolished the leptin-induced increase in body temperature in both cases. Mice lacking (gene knockout) the main IL-1 receptor (80 kDa, R1) responsible for IL-1 actions showed no reduction in food intake in response to leptin. These data indicate that leptin actions in the brain depend on IL-1, and we show further that the effect of leptin on fever, but not food intake, is abolished by a cyclooxygenase inhibitor. Thus, we propose that in addition to its role in body weight regulation, leptin may mediate neuroimmune responses via actions in the brain dependent on release of IL-1 and prostaglandins.

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.