The expressions of mRNAs for interleukin-6 (IL-6) and the IL-6 receptor (IL-6R) in the rat hypothalamus and midbrain during restraint stress

Peripheral IL-6/STAT3 signaling promotes beiging of white fat

Interleukin-6 (IL-6) can reportedly centrally affect the thermogenesis of brown fat. However, whether the peripheral IL-6 signaling regulates beiging of white fat remains largely unknown. In vitro experiments indicated IL-6-KO-derived white adipocytes exhibited lower thermogenic gene expression compared to the WT, associating with reduced phosphorylation of STAT3 at Tyr705. Mechanistically, exogenous IL-6 application increased the p-STAT3^Tyr705 level, thus the phosphorylated STAT3 bound to the promoter regions, and enhanced the transcription of Pparγ and Ucp1. The protein interaction of PGC-1α with PPARγ was increased by IL-6, which also contributed to stimulate Ucp1 expression. In vivo experiments demonstrated that IL-6 KO decreased the beiging potential of white fat with suppressed STAT3 Tyr705 phosphorylation. Accordingly, IL-6-KO mature mice were associated with disrupted glucose homeostasis and accelerated hepatic steatosis. Collectively, we identified a novel function of peripheral IL-6/STAT3 signaling which is essential for beiging of white fat, such ensuring fat and glucose homeostasis.

Noradrenaline as a key neurotransmitter in modulating microglial activation in stress response

State of mind can influence susceptibility and progression of diseases and disorders not only in peripheral organs, but also in the central nervous system (CNS). However, the underlying mechanism how state of mind can affect susceptibility to various illnesses in the CNS is not fully understood. Among a number of candidates responsible for stress-induced neuroimmunomodulation, noradrenaline has recently been shown to play crucial roles in the major immune cells of the brain, microglia. In particular, recent studies have demonstrated that noradrenaline may be a key neurotransmitter in modulating microglial cells, thereby determining different cell conditions and responses ranging from resting to activation state depending on host stress level or whether the host is awake or asleep. For instance, microglia under resting conditions may have constructive roles in surveillance, such as debris clearance, synaptic monitoring, pruning, and remodeling. In contrast, once activated, microglia may become less efficient in surveillance activities, and instead implicated in detrimental roles such as cytokine or superoxide release. It is also likely that glial activation, both astrocytes and microglia, are negatively associated with the clearance of brain waste via the glymphatic system. In this review, we discuss the possible underlying mechanism as well as the roles of stress-induced microglial activation.

Interleukin-6 in Schizophrenia-Is There a Therapeutic Relevance?

Renewing interest in immune aspects of schizophrenia and new findings about the brain-fat axis encourage us to discuss the possible role of interleukin-6 (IL-6) in schizophrenia. Previously, it was suggested that a primary alteration of the innate immune system may be relevant in schizophrenia. Functional dichotomy of IL-6 suggests that this chemical messenger may be responsible for regulating the balance between pro- and anti-inflammatory responses, with tissue-specific properties at the periphery and in the central nervous system. Specific phase of this chronic and deteriorating disorder must be considered, which can involve IL-6 in acute or possible chronic inflammation and/or autoimmunity. We give an overview of IL-6 role in the onset and progression of this disorder, also considering cognitive impairment and metabolic changes in patients with schizophrenia. Data suggest that decreased serum level of IL-6 following antipsychotic therapy could be predisposing factor for the development of obesity and obesity-related metabolic disorders in schizophrenia. As we reviewed, the IL-6 plays significant role in disease genesis and progression, so the use of specific inhibitors may not only be beneficial for exacerbation and alleviation of positive symptoms, but may attenuate cognitive impairment in patients with schizophrenia.

Divergent responses of inflammatory mediators within the amygdala and medial prefrontal cortex to acute psychological stress

There is now a growing body of literature that indicates that stress can initiate inflammatory processes, both in the periphery and brain; however, the spatiotemporal nature of this response is not well characterized. The aim of this study was to examine the effects of an acute psychological stress on changes in mRNA and protein levels of a wide range of inflammatory mediators across a broad temporal range, in key corticolimbic brain regions involved in the regulation of the stress response (amygdala, hippocampus, hypothalamus, medial prefrontal cortex). mRNA levels of inflammatory mediators were analyzed immediately following 30min or 120min of acute restraint stress and protein levels were examined 0h through 24h post-termination of 120min of acute restraint stress using both multiplex and ELISA methods. Our data demonstrate, for the first time, that exposure to acute psychological stress results in an increase in the protein level of several inflammatory mediators in the amygdala while concomitantly producing a decrease in the protein level of multiple inflammatory mediators within the medial prefrontal cortex. This pattern of changes seemed largely restricted to the amygdala and medial prefrontal cortex, with stress producing few changes in the mRNA or protein levels of inflammatory mediators within the hippocampus or hypothalamus. Consistent with previous research, stress resulted in a general elevation in multiple inflammatory mediators within the circulation. These data indicate that neuroinflammatory responses to stress do not appear to be generalized across brain structures and exhibit a high degree of spatiotemporal specificity. Given the impact of inflammatory signaling on neural excitability and emotional behavior, these data may provide a platform with which to explore the importance of inflammatory signaling within the prefrontocortical-amygdala circuit in the regulation of the neurobehavioral responses to stress.

Cancer as a Proinflammatory Environment: Metastasis and Cachexia

The development of the syndrome of cancer cachexia and that of metastasis are related with a poor prognostic for cancer patients. They are considered multifactorial processes associated with a proinflammatory environment, to which tumour microenvironment and other tissues from the tumour bearing individuals contribute. The aim of the present review is to address the role of ghrelin, myostatin, leptin, HIF, IL-6, TNF-α, and ANGPTL-4 in the regulation of energy balance, tumour development, and tumoural cell invasion. Hypoxia induced factor plays a prominent role in tumour macro- and microenvironment, by modulating the release of proinflammatory cytokines.

The expression of interleukin-6 and its receptor in various brain regions and their roles in exploratory behavior and stress responses

We examined the involvement of interleukin-6 (IL-6) and its receptor IL-6Rα on behavior and stress responses in mice. In the open field, both wild-type (WT) and IL-6 deficient mice displayed similar levels of locomotor activity; however, IL-6 deficient mice spent more time in the central part of the arena compared to control WT mice. After behavioral testing, mice were subjected to stress and then sacrificed. The levels of IL-6 and its receptor in their brains were determined. Immunohistochemical labeling of brain sections for IL-6 showed a high level of expression in the subventricular zone of the lateral ventricles and in the border zone of the third and fourth ventricles. Interestingly, 95% of the IL-6-expressing cells had an astrocytic phenotype, and the remaining 5% were microglial cells. A low level of IL-6 expression was observed in the olfactory bulb, hypothalamus, hippocampus, cerebral cortex, cerebellum, midbrain and several brainstem structures. The vast majority of IL-6-expressing cells in these structures had a neuronal phenotype. Stress increased the number of IL-6-immunoreactive astrocytes and microglial cells. The levels of the IL-6Rα receptor were increased in the hypothalamus of stressed mice. Therefore, in this study, we describe for the first time the distribution of IL-6 in various types of brain cells and in previously unreported regions, such as the subventricular zone of the lateral ventricle. Moreover, we provide data on regional distribution and expression within specific cell phenotypes. This highly differential expression of IL-6 indicates its specific roles in the regulation of neuronal and astrocytic functions, in addition to the roles of IL-6 and its receptor IL-6Rα in stress responses.

The effects of central pro-and anti-inflammatory immune challenges on depressive-like behavior induced by chronic forced swim stress in rats

Although increasing evidence demonstrates that both chronic stressors and inflammatory immune activation contribute to pathophysiology and behavioral alterations associated with major depression, little is known about the interaction effect of central inflammatory immune activation and stress on depressive-like behavior. Our previous work has shown that 14-day chronic forced swim stress induces significant depressive-like behavior. The present investigation assessed whether pro-inflammatory cytokine and anti-inflammatory cytokine challenges have differential interaction effect on depressive-like behavior induced by chronic forced swim stress in rats. The pro-inflammatory and anti-inflammatory immune challenges were achieved respectively by central administration of lipopolysaccharide (LPS), a pro-inflammatory cytokine inducer, and interleukin-10 (IL-10), an anti-inflammatory cytokine. It was found that either central LPS treatment alone or chronic forced swim stress alone significantly induced depressive-like behavior, including reduced body weight gain, reduced saccharin preference and reduced locomotor activity. However, there was no significant synergistic or additive effect of central LPS treatment and stress on depressive-like behavior. LPS treatment did not exacerbate the depressive-like behavior induced by forced swim stress. Nevertheless, IL-10 reversed depressive-like behavior induced by forced swim stress, a finding indicating that IL-10 has antidepressant effect on behavioral depression induced by stress. The present findings provide new insight into the complexity of the immunity-inflammation hypothesis of depression.

Corticosteroids limit microglial activation occurring during acute stress

Our previous studies demonstrated that exposure of animals to acute stress immediately induced morphological microglial activation in the brain. Here we investigated the effects of adrenal corticoids on microglial activation following acute stress. We compared microglial activation in vivo in adrenalectomized (ADX), Sham-operated (SHM), and adrenalectomy plus corticosterone (CORT) administered rats exposed to a 2-h period of acute water restraint stress. We found that: (1) acute stress induced microglial activation in SHM rats; (2) acute stress robustly enhanced microglial activation in ADX rats; (3) CORT treatment significantly reduced the effects of adrenalectomy. Thus, while acute stress has the ability to activate microglia, the magnitude of activation is negatively regulated by CORT. Glucocorticoids may serve as an important endogenous suppressive signal limiting neuroinflammation that might otherwise occur during stress.

Evidence for sustained elevation of IL-6 in the CNS as a key contributor of depressive-like phenotypes

There is compelling clinical literature implicating a role for cytokines in the pathophysiology of major depressive disorder (MDD). Interleukin-6 (IL-6) and interleukin-1β (IL-1β) are pleiotropic inflammatory cytokines that have been reported to be elevated in patients with MDD. The present studies were undertaken to investigate the relationship between IL-6 and IL-1β in animal models of depressive-like behavior. Analysis of brain tissue homogenates in the cortex of rats subjected to chronic stress paradigms revealed elevated levels of IL-6 protein in the absence of elevations in IL-1β. Central administration of recombinant mouse IL-6 produced depressive-like phenotypes in mice, which were not accompanied by IL-1β-induced increases in the brain tissue or IL-1β-related sickness behavior typical of a general central nervous system inflammatory response. Systemic administration of fluoxetine in the presence of centrally administered IL-6 failed to produce the expected antidepressant-like response in mice relative to sham-infused controls. Further, administration of fluoxetine to mice with endogenous overexpression of brain IL-6 (MRL/MpJ-Fas(LPR/LPR) (LPR mice)) failed to produce the expected antidepressant-like effect relative to fluoxetine-treated control mice (MRL/MpJ(+/+)). Interestingly, blockade of IL-6 trans-signaling by coadministration of a gp130/Fc monomer or an anti-mouse IL-6 antibody with IL-6 prevented the IL-6-induced increases in immobility time as well as attenuated IL-6-induced increases of protein in the cortex. Taken together, these data indicate that elevations in IL-6 may have a pathophysiological role underlying depression and more specifically resistance to current classes of antidepressant medications and suggest that modulation of the IL-6 signaling pathway may have therapeutic potential for treatment-resistant depression.

Interleukin-6 regulates the expression of hypothalamic neuropeptides involved in body weight in a gender-dependent way

Interleukin (IL)-6 has been involved in the control of body weight and body fat. Nevertheless, the mechanisms underlying these effects are not completely understood because central and peripheral actions of IL-6 are plausible. To gain further insight into the central effects of IL-6, we used transgenic mice expressing the IL-6 gene under the control of the glial fibrillary acidic protein (GFAP) promoter (GFAP-IL-6 mice), therefore with central nervous system-restricted over-expression of IL-6, and we studied the expression of the main neuropeptides responsible for energy homeostasis in specific hypothalamic nuclei. Neuropeptide Y (NPY), agouti-related peptide (AgRP), melanin-concentrating hormone (MCH), prepro-orexin (preproOX) (orexigenic and anabolic neuropeptides), pro-opiomelanocortin (POMC) and corticotrophin-releasing hormone (CRH) (anorexigenic and catabolic peptides) mRNA levels were determined using in situ hybridisation in young (2-4 month-old) and old (10-12 month-old) female and male mice under different feeding conditions: normal diet (control) and high-fat diet (HFD), and 24 h-food deprivation. In GFAP-IL-6 females fed a control diet (GFAP-IL-6-control), we showed a significant decrease in NPY and AgRP mRNA levels at all ages, and a late increase in POMC expression (only significant in older animals). These differences were blunted in HFD mice. By contrast, GFAP-IL-6-control males showed a decrease in CRH mRNA content at early ages (2-4 months), and an increase in older mice (10-12 months). Interestingly, these differences were again blunted in HFD mice. Finally, central IL-6 was not able to counteract the effects of 24 h of fasting on body weight, plasma glucose levels and the mRNA content of the peptides evaluated in the present study. Our results demonstrate that IL-6 may regulate the expression of hypothalamic neuropeptides involved in the control of body weight and body fat acting at the central level in a gender- and age-dependent way.

Attention-deficit hyperactivity disorder (ADHD) and glial integrity: an exploration of associations of cytokines and kynurenine metabolites with symptoms and attention

Background

In contrast to studies of depression and psychosis, the first part of this study showed no major differences in serum levels of cytokines and tryptophan metabolites between healthy children and those with attention-deficit/hyperactivity disorder of the combined type (ADHD). Yet, small decreases of potentially toxic kynurenine metabolites and increases of cytokines were evident in subgroups. Therefore we examined predictions of biochemical associations with the major symptom clusters, measures of attention and response variability.

Methods

We explored systematically associations of 8 cytokines (indicators of pro/anti-inflammatory function) and 5 tryptophan metabolites with symptom ratings (e.g. anxiety, opposition, inattention) and continuous performance test (CPT) measures (e.g. movement, response time (RT), variability) in 35 ADHD (14 on medication) and 21 control children. Predictions from linear regressions (controlled by the false discovery rate) confirmed or disconfirmed partial correlations accounting for age, body mass and socio-economic status.

Results

(1) Total symptom ratings were associated with increases of the interleukins IL-16 and IL-13, where relations of IL-16 (along with decreased S100B) with hyperactivity, and IL-13 with inattention were notable. Opposition ratings were predicted by increased IL-2 in ADHD and IL-6 in control children. (2) In the CPT, IL-16 related to motor measures and errors of commission, while IL-13 was associated with errors of omission. Increased RT variability related to lower TNF-α, but to higher IFN-γ levels. (3) Tryptophan metabolites were not significantly related to symptoms. But increased tryptophan predicted errors of omission, its breakdown predicted errors of commission and kynurenine levels related to faster RTs.

Conclusions

Many associations were found across diagnostic groups even though they were more marked in one group. This confirms the quantitative trait nature of these features. Conceptually the relationships of the pro- and antiinflammatory cytokines distinguished between behaviours associated more with cognitive or more with motor control respectively. Further study should extend the number of immunological and metabolic markers to confirm or refute the trends reported here and examine their stability from childhood to adolescence in a longitudinal design.

Expression of interleukin 6 in brain and colon of rats with TNBS-induced colitis

AIM: To characterise expression of interleukin 6 (IL-6), a potent proinflammatory cytokine, in the occurrence and development of inflammatory bowel disease (IBD) and investigate its effect on neuroimmunomodulation and immune homeostasis regulation.

METHODS: In this study, rats with colitis induced by trinitrobenzene sulfonic acid (TNBS) were sacrificed on days 3, 7, 14, 21 and 28 after induction. In the controls, the TNBS was just replaced by equivalent amount of phosphate buffered solution (PBS, 0.01 mol/L). IL-6 mRNA expression in brain and colon tissues in each phase was evaluated by real-time reverse transcription-polymerase chain reaction, and cellular localisation and protein level of IL-6 was determined by immunohistochemistry.

RESULTS: At day 7, mRNA expression of IL-6 was significantly higher in the colon and brain of IBD rats than that of the controls. The protein level was also significantly higher in colon, hypothalamus and cerebral cortex of IBD rats compared with the controls. So there are similar temporal trends in IL-6 mRNA expression and protein levels in all positions with a persistent increase to a peak at day 7, followed by a decline and gradual return to normal levels.

CONCLUSION: These results revealed that changes in IL-6 expression in brain and colon tissues occur in different phases of IBD. Therefore, we propose that the nerve centre regulates and controls the occurrence and development of IBD via IL-6.

Interleukin-6 as a mechanism for the adverse effects of social stress on acute Theiler's virus infection

Prior exposure to social disruption stress (SDR) exacerbates both the acute and chronic phase of Theiler's murine encephalomyelitis virus infection (TMEV; [Johnson, R.R., Storts, R., Welsh, T.H., Jr., Welsh, C.J., Meagher, M.W., 2004. Social stress alters the severity of acute Theiler's virus infection. J. Neuroimmunol. 148, 74--85; Johnson, R.R., Prentice, T.W., Bridegam, P., Young, C.R., Steelman, A.J., Welsh, T.H., Welsh, C.J.R., Meagher, M.W., 2006. Social stress alters the severity and onset of the chronic phase of Theiler's virus infection. J. Neuroimmunol. 175, 39--51]). However, the neuroimmune mechanism(s) mediating this effect have not been determined. The present study examined whether stress-induced increases in the proinflammatory cytokine interleukin-6 (IL-6) contributes to the adverse effects of SDR on acute TMEV infection. Experiment 1 demonstrated that SDR increases central and peripheral levels of IL-6 and that this effect is reversed by intracerebral ventricular infusion of neutralizing antibody to IL-6 prior to each of six SDR sessions. Although SDR reduced the sensitivity of spleen cells to the anti-inflammatory effects of corticosterone, the neutralizing antibody to IL-6 did not alter this effect. To investigate whether stress-induced increases in IL-6 contribute to the exacerbation of acute TMEV infection, Experiment 2 examined whether intracerebral administration of neutralizing antibody to IL-6 during SDR would prevent the subsequent exacerbation of acute TMEV infection. Experiment 3 then replaced the social stress with intracerebral infusion of IL-6 to assess sufficiency. As expected, prior exposure to SDR subsequently increased infection-related sickness behaviors, motor impairment, CNS viral titers, and CNS inflammation. These deleterious effects of SDR were either prevented or significantly attenuated by intracerebral infusion of neutralizing antibody to IL-6 during the stress exposure period. However, infusion of IL-6 alone did not mimic the adverse effects of SDR. We conclude that IL-6 is necessary but not sufficient to exacerbate acute TMEV infection.

Social conflict exacerbates an animal model of multiple sclerosis

A growing body of evidence suggests that social conflict is associated with inflammatory disease onset and exacerbations in multiple sclerosis (MS) patients and in animal models of MS. This review illustrates how animal research can be used to elucidate the biobehavioral mechanisms underlying the adverse health effects of social conflict. The authors review studies indicating that social conflict exacerbates a virally initiated animal model of MS. This research suggests that the deleterious effects of social conflict may be partially mediated by stress-induced increases in pro-inflammatory cytokine levels in the central nervous system. In addition, they provide evidence that the adverse health effects of social conflict can be prevented by blocking the stress-induced increases in cytokine activity. This suggests that interventions designed to prevent or reverse the stress-induced increases in cytokine activity may be able to prevent or reverse some of the negative health effects of social conflict in humans.

Prior intraperitoneal injection of rat recombinant IL-6 increases hypothalamic IL-6 contents in subsequent forced swim stressor in rats

BACKGROUND

Cytokines do not only mediate responses to infection, but are also involved in behavioral and physiological responses to psychological stressors. IL-6 has received special attention in relation to the development of posttraumatic stress disorders and depression.

OBJECTIVE

We tested effects of prior injection of rat recombinant IL-6 (rrIL-6) on behaviors induced by the forced swim (FS) stressor, and central and peripheral responses of IL-6 to FS.

METHODS

Rats were injected intraperitoneally with either rrIL-6 (250 ng/0.5 ml) or equal-volume sterile saline twice within an interval of 24 h. One hour after each injection, the rats were exposed to FS or remained at the home cage (control).

RESULTS

Injection of rrIL-6 did not affect immobility, swimming or climbing behaviors during FS compared with the saline control. Although FS was not a significant factor for hypothalamic and midbrain IL-6 mRNA and plasma IL-6 responses, FS with prior administration of rrIL-6 significantly increased hypothalamic IL-6 contents in response to FS compared with the saline injection-FS condition.

CONCLUSIONS

Our results suggested that stressor alone had no influence on plasma IL-6 levels and IL-6 mRNA expression levels in midbrain and hypothalamus, but administration of rrIL-6 followed by FS significantly increased hypothalamic IL-6. Our results support the notion that the interaction between IL-6 and stressor might have implications for the pathophysiology of IL-6-induced depressive symptoms.

Stress induced morphological microglial activation in the rodent brain: involvement of interleukin-18

The present study investigated the possibility that acute stress might activate microglial cells. Wistar rats were exposed to 2 h period of restraint combined with water immersion stress prior to brain analysis by immunohistochemistry with OX-42, a marker of complement receptor CR3. A single session of stress provoked robust morphological microglial activation in the thalamus, hypothalamus, hippocampus, substantia nigra and central gray. These effects appeared as early as at 1 h of exposure and were further intensified at 2 h. Morphological activation was not accompanied with changes in markers of functional activation or of inflammation including interleukin-1beta (IL-1beta), interleukin-6 (IL-6) and inducible nitric oxide synthase (iNOS). Similar results were obtained with mice where the effects of stress were compared in animals null for interleukin-18 (IL-18 KO), a cytokine previously demonstrated to be modulated by stress and to contribute to microglia activation. The results demonstrated significant reduction of stress-induced microglial activation in IL-18 KO mice. The present study reports evidence that physical/emotional stress may induce morphological microglial activation in the brain and this activation is in part mediated by interleukin-18.

Appetite and energy balance signals from adipocytes

Interest in the biology of white adipose tissue has risen markedly with the recent surge in obesity and its associated disorders. The tissue is no longer viewed simply as a vehicle for lipid storage; instead, it is recognized as a major endocrine and secretory organ. White adipocytes release a multiplicity of protein hormones, signals and factors, termed adipokines, with an extensive range of physiological actions. Foremost among these various adipokines is the cytokine-like hormone, leptin, which is synthesized predominantly in white fat. Leptin plays a critical role in the control of appetite and energy balance, with mutations in the genes encoding the hormone or its receptor leading to profound obesity in both rodents and man. Leptin regulates appetite primarily through an interaction with hypothalamic neuroendocrine pathways, inhibiting orexigenic peptides such as neuropeptide Y and orexin A, and stimulating anorexigenic peptides such as proopiomelanocortin. White fat also secretes several putative appetite-related adipokines, which include interleukin-6 and adiponectin, but whether these are indeed significant signals in the regulation of food intake has not been established. Through leptin and the other adipokines it is evident that adipose tissue communicates extensively with other organs and plays a pervasive role in metabolic homeostasis.

Combined interleukin-6 and interleukin-1 deficiency causes obesity in young mice

Proinflammatory cytokines including interleukin (IL)-1 and IL-6 exert pleiotropic effects on the neuro-immuno-endocrine system. Previously, we showed that IL-1 receptor antagonist-deficient (IL-1Ra(-/-)) mice show a lean phenotype due to an abnormal lipid metabolism. On the contrary, it was reported that IL-6(-/-) mice exhibit obesity after 6 months of age. This study sought to assess the roles of IL-1 and IL-6 in body weight homeostasis. We generated mice deficient in IL-6 and IL-1Ra (IL-6(-/-) IL-1Ra(-/-)) and IL-6, IL-1alpha, and IL-1beta (IL-6(-/-) IL-1(-/-)). IL-6(-/-) IL-1Ra(-/-) mice exhibited a lean phenotype, similar to IL-1Ra(-/-) mice. On the other hand, IL-6(-/-) IL-1(-/-) mice became obese as early as 10 weeks of age, while IL-1(-/-) mice and IL-6(-/-) mice were normal at this age. The daily food intake was significantly higher in IL-6(-/-) IL-1(-/-) mice than in IL-6(-/-) IL-1(+/-) mice, while energy expenditure was comparable in these two strains. Acute anorexia induced by peripheral administration of IL-1 was significantly suppressed in IL-6(-/-) IL-1(-/-) mice, but not in IL-1(-/-) mice or IL-6(-/-) mice compared with wild-type mice. These results indicate that IL-1 and IL-6 are both involved in the regulation of body fat in a redundant manner in young mice.

Developmental exposure to corticosterone: behavioral changes and differential effects on leukemia inhibitory factor (LIF) and corticotropin-releasing hormone (CRH) gene expression in the mouse

RATIONALE

Cytokines are found in both the peripheral and central nervous system. There has been increasing interest in their potential role in some of the behavioral features of depressive disorders. Leukemia inhibitory factor (LIF), a proinflammatory cytokine, produces stimulation of adrenocorticotropic hormone (ACTH) secretion in response to emotional and inflammatory stress and recently has been linked to depressive-type behavior. Both the hypothalamic-pituitary-adrenal axis and the immune system, including cytokine-mediated responses, appear to be susceptible to long-term programming during fetal and neonatal development.

OBJECTIVE

The present study was designed to characterize the effects of perinatal exposure to corticostereone on behavior, hypothalamic LIF and corticotropin-releasing hormone (CRH) mRNA expression, and basal plasma corticosterone levels in adult female mice.

METHODS

Corticosterone was added to the drinking water beginning the last week of gestation and continued until weaning. Behavior in the open field and forced swim tests, baseline plasma corticosterone levels, and hypothalamic LIF and CRH gene expression were evaluated in the adult offspring.

RESULTS

Mice exposed to perinatal corticosterone showed increased immobility in the forced swim test and increased locomotor activity in the open field test. Although there were no differences between treatment groups in terms of basal plasma levels of corticosterone or hypothalamic CRH mRNA, LIF mRNA expression was increased in the hypothalamus.

CONCLUSIONS

These results show that perinatal exposure to glucocorticoids can produce long-term behavioral changes and upregulation of central LIF mRNA expression.

Reduced immobility in the forced swim test in mice with a targeted deletion of the leukemia inhibitory factor (LIF) gene

Cytokines are a large and diverse group of polypeptides that are rapidly released in response to tissue injury, infection, and inflammation. Besides their effects in the periphery, cytokines also affect the central nervous system (CNS). There has been increasing interest in the potential role of cytokines in the behavioral features of depressive disorders. One cytokine that might be a candidate for a role in the etiology of depression is leukemia inhibitory factor (LIF). LIF mRNA has been detected in the hypothalamus, hippocampus, amygdala, cerebellum, cerebral cortex, and basal forebrain nuclei. The role of LIF in the CNS has not been fully elucidated. Based upon the hypothesis that cytokines might have a role in depression, the present study characterized the behavior of mice with a targeted disruption of the LIF gene (LIF knockouts) in the forced swim test, an animal model used to measure depressive-like behavior and the response to antidepressants. It was found that LIF knockout mice show reduced immobility in the forced swim test, suggesting that LIF might have a potential role in the etiology of some forms of depression.

Cerebral perturbations provoked by prolonged exercise

This review addresses cerebral metabolic and neurohumoral alterations during prolonged exercise in humans with special focus on associations with fatigue. Global energy turnover in the brain is unaltered by the transition from rest to moderately intense exercise, apparently because exercise-induced activation of some brain regions including cortical motor areas is compensated for by reduced activity in other regions of the brain. However, strenuous exercise is associated with cerebral metabolic and neurohumoral alterations that may relate to central fatigue. Fatigue should be acknowledged as a complex phenomenon influenced by both peripheral and central factors. However, failure to drive the motorneurons adequately as a consequence of neurophysiological alterations seems to play a dominant role under some circumstances. During exercise with hyperthermia excessive accumulation of heat in the brain due to impeded heat removal by the cerebral circulation may elevate the brain temperature to >40 degrees C and impair the ability to sustain maximal motor activation. Also, when prolonged exercise results in hypoglycaemia, perceived exertion increases at the same time as the cerebral glucose uptake becomes low, and centrally mediated fatigue appears to arise as the cerebral energy turnover becomes restricted by the availability of substrates for the brain. Changes in serotonergic activity, inhibitory feed-back from the exercising muscles, elevated ammonia levels, and alterations in regional dopaminergic activity may also contribute to the impaired voluntary activation of the motorneurons after prolonged and strenuous exercise. Furthermore, central fatigue may involve depletion of cerebral glycogen stores, as signified by the observation that following exhaustive exercise the cerebral glucose uptake increases out of proportion to that of oxygen. In summary, prolonged exercise may induce homeostatic disturbances within the central nervous system (CNS) that subsequently attenuates motor activation. Therefore, strenuous exercise is a challenge not only to the cardiorespiratory and locomotive systems but also to the brain.

Peripheral and central proinflammatory cytokine response to a severe acute stressor

The role of proinflammatory cytokines in the response to acute stressor exposure has received recent attention. Exposure to a single session of inescapable shock (IS) induces peripheral and central proinflammatory cytokines. Other stressors also increase expression of proinflammatory cytokine mRNA and/or protein in various tissues. However, the induction of central and peripheral proinflammatory cytokines by stressors remains controversial and the pattern of cytokine induction is not consistent across stressors. The present experiments sought to examine the pattern of the proinflammatory cytokine response to a stressor known to cause elevations of IL-1beta protein. mRNA expression for three proinflammatory cytokines, IL-1beta, TNF-alpha and IL-6, and IL-1beta protein was examined after IS. IS increases IL-1beta mRNA and/or protein in a variety of tissues, including hypothalamus, hippocampus, pituitary and spleen. Furthermore, IS concomitantly alters IL-1beta mRNA and protein in hypothalamus and spleen, while the IL-1beta mRNA increase in pituitary lags behind the increase of IL-1beta protein. Interestingly, IL-1beta mRNA is elevated in hippocampus 4 h after IS, but an increase of IL-1beta protein in hippocampus is not detected. Expression of TNF-alpha and IL-6 mRNA do not increase in response to IS. Indeed, TNF-alpha mRNA expression decreases in cortex, pituitary and liver immediately after IS. These findings suggest that alterations of proinflammatory cytokine expression by stressors, and IS in particular, are region- and cytokine-specific.

Chronic mild stress in mice decreases peripheral cytokine and increases central cytokine expression independently of IL-10 regulation of the cytokine network

OBJECTIVES

Accumulating evidence indicates that stress leads to an increased expression of pro-inflammatory cytokines such as interleukin (IL)-6. The production and action of pro-inflammatory cytokines are down-regulated by anti-inflammatory cytokines such as IL-10. This makes IL-10-deficient mice a potentially useful model to assess the effects of stress on cytokine production.

METHODS

In the present study, IL-10-deficient mice were compared to wild-type mice in their behavioural and cytokine response to a chronic mild stress procedure.

RESULTS

The 3-week chronic mild stress decreased body weight gain and sucrose consumption. It also resulted in a decreased expression of peripheral IL-1beta and IL-6 and an increased expression of brain IL-6. This last change in IL-6 was correlated to body weight loss in stressed mice. However, IL-10-deficient mice did not differ from wild-type mice in their response to the chronic mild stress procedure, despite substantial differences in functioning of the cytokine network.

CONCLUSION

These results are interpreted in the context of the relationship between cytokines and behaviour.

On the site and mechanism of action of the anti-obesity effects of interleukin-6

We conducted an experimental study examining the site and mechanism of action of the anti-obesity effect of interleukin-6 (IL-6) in mice and rats. We used dual energy X-ray absorptiometry (DEXA) and computerized tomography to investigate the body composition of mice with knockout of the IL-6 gene and wild-type control mice. Rats were treated with IL-6 or vehicle through intracerebroventricular (ICV) cannulae. Energy expenditure was measured as oxygen consumption by indirect calorimetry in metabolic chambers. Results showed that the mice lacking IL-6 increased in body weight compared with wild-type mice from 6 months of age onward, although there was no marked difference in food intake between the pre-obese IL-6 knockout mice and the wild-type mice. IL-6 given as a single ICV injection to rats stimulated oxygen consumption; whereas, the same doses were ineffective when given peripherally. Chronic ICV IL-6 treatment decreased body weight and fat mass in rodents. Administration of IL-6 may decrease fat mass in mice and rats by stimulating energy expenditure at the CNS level, possibly in the hypothalamus.

Interleukin-6 release from the human brain during prolonged exercise

Interleukin (IL)-6 is a pleiotropic cytokine, which has a variety of physiological roles including functions within the central nervous system. Circulating IL-6 increases markedly during exercise, partly due to the release of IL-6 from the contracting skeletal muscles, and exercise-induced IL-6 may be linked with central fatigue, which is enhanced by hyperthermia. Exercise-induced IL-6 may also stimulate hepatic glycogenolysis, which is important during prolonged and repeated exercise. Thus, in a randomised order and separated by 60 min of rest, eight young male subjects completed two 60 min exercise bouts: one bout with a normal (38 degrees C) and the other with an elevated (39.5 degrees C) core temperature. The cerebral IL-6 response was determined on the basis of internal jugular venous to arterial IL-6 differences and global cerebral blood flow. There was no net release or uptake of IL-6 in the brain at rest or after 15 min of exercise, but a small release of IL-6 was observed after 60 min of exercise in the first bout (0.06 +/- 0.03 ng min(-1)). This release of IL-6 from the brain was five-fold greater at the end of the second bout (0.30 +/- 0.08 ng min(-1); P < 0.05) with no separate influence of hyperthermia. In conclusion, IL-6 is released from the brain during prolonged exercise in humans and it appears that the duration of the exercise rather than the increase in body temperature dictates the cerebral IL-6 response.

Intracerebroventricular interleukin-6 treatment decreases body fat in rats

Recently we found that interleukin-6 (IL-6) knockout mice develop mature-onset obesity and that a single intracerebroventricular (ICV) injection of IL-6 increases energy expenditure. In the present study we investigated if chronic ICV treatment with IL-6 can suppress body fat mass. IL-6 was injected ICV daily for two weeks to rats fed a high-fat diet. IL-6 treatment but not saline treatment decreased body weight by 8.4% and decreased the relative weights of mesenteric and retroperitoneal fat pads. Consistent with this, circulating leptin levels were decreased by 40% after IL-6 treatment but not after saline treatment. Average food intake per day was decreased in the IL-6 treated group compared to the saline treated rats. IL-6 treatment did not change hepatic expression of the acute-phase protein haptoglobin, serum levels of insulin or insulin-like growth factor-I, or the weights of the heart, liver, kidneys, adrenals, and spleen. We conclude that centrally administered IL-6 can decrease body fat in rats without causing acute-phase reaction.

Interleukin-6-deficient mice develop mature-onset obesity

The immune-modulating cytokine interleukin-6 (IL-6) is expressed both in adipose tissue and centrally in hypothalamic nuclei that regulate body composition. We investigated the impact of loss of IL-6 on body composition in mice lacking the gene encoding IL-6 (Il6-/- mice) and found that they developed mature-onset obesity that was partly reversed by IL-6 replacement. The obese Il6-/- mice had disturbed carbohydrate and lipid metabolism, increased leptin levels and decreased responsiveness to leptin treatment. To investigate the possible mechanism and site of action of the anti-obesity effect of IL-6, we injected rats centrally and peripherally with IL-6 at low doses. Intracerebroventricular, but not intraperitoneal IL-6 treatment increased energy expenditure. In conclusion, centrally acting IL-6 exerts anti-obesity effects in rodents.

IL-6 up-regulates CNTF mRNA expression and enhances neurite regeneration

Interleukin-6 (IL-6) is a neurotrophic cytokine, however, its direct effect on nerve regeneration has not been well characterized. We therefore examined the effect of IL-6 on neurite regeneration using the rat dorsal root ganglion. IL-6 significantly enhanced neurite regeneration from transected nerve terminals. We also examined the mRNA expression of IL-6 family cytokines and their receptors during the regeneration. The mRNA expressions of IL-6, IL-6 receptor, leukemia inhibitory factor (LIF), ciliary neurotrophic factor (CNTF) receptor alpha, and LIF receptor beta showed no significant differences by the addition of IL-6. In contrast, IL-6 enhanced the mRNA expression of gp130 and CNTF. In addition, CNTF significantly increased neurite regeneration when added exogenously. Our data suggest that IL-6 enhanced regeneration via up-regulating CNTF expression.

Plasma corticosterone and immune reactivity in restrained female C3H mice

Psychological stressors are known to stimulate the hypothalamic-pituitary-adrenal axis and the sympathetic nervous system resulting in the release of corticosterone and catecholamines respectively. They have also been reported to induce cytokine production. All these molecules affect various immune parameters and can alter overall immune competence of the individual. The purpose of this investigation was to study the regulation of the production of corticosterone during stress and its possible effects on immune reactivity. In a first series of experiments, the possible regulation of corticosterone production by interleukin (IL)-1beta and peripheral catecholamines during restraint was assessed using a pharmacological approach in mice. Plasma IL-1beta concentrations remained at basal after 1-h restraint and the stress-induced increase of plasma corticosterone was not modified by a peripheral injection of an IL-1 receptor antagonist (IL-1ra). By contrast, chemical sympathectomy potentiated the restraint-induced increase in plasma corticosterone concentration, this potentiation being reversed by IL-1ra. In a second series of experiments, the role of corticosterone in stress-immune relationships was studied in adrenalectomized mice subjected to restraint and immunized with sheep erythrocytes. Non-specific immunity, i.e. proliferation of splenocytes and thymocytes and plasma levels of IL-1beta, as well as specific immunity, i.e. antibody production and delayed hypersensitivity, were not altered after 2-h restraint. Adrenalectomy failed to induce immune effects in stressed animals, except that delayed hypersensitivity was stronger in adrenalectomized animals, revealing that the high levels of corticosterone produced during stress have an anti-inflammatory activity. The present data show that the stress-induced production of corticosterone was modulated by both peripheral catecholamines and IL-1beta. However, this production of corticosterone was unable to modulate immune reactivity except delayed hypersensitivity.

Effects of repeated stress on expression of interleukin-6 (IL-6) and IL-6 receptor mRNAs in rat hypothalamus and midbrain

We examined the effects of single and repeated stress on the expression of interleukin-6 (IL-6) and IL-6 receptor (IL-6R) mRNAs in the rat midbrain and hypothalamus using reverse transcriptase-polymerase chain reaction (RT-PCR). Following a single episode of restraint stress for 4 hours (1R) or 4 hours per day on two (2R) or three (3R) consecutive days, the hypothalamus and midbrain were removed immediately and the levels of IL-6 and IL-6R mRNAs in both regions were determined. Regional differences in stress-related changes in mRNA levels were noted. The expression of IL-6 mRNA in the hypothalamus did not change in 1R group but decreased in 2R and 3R groups. The expression of IL-6R mRNA in the same region significantly diminished in all groups. In the midbrain, the expression of IL-6 mRNA increased in 1R group and decreased in 2R and 3R, while the expression of IL-6R mRNA significantly diminished in 1R and 3R groups but was not different from control in 2R group. Our findings indicate that repeated stress in rats produce changes in IL-6 and IL-6R mRNAs in the midbrain and hypothalamus that are different than those of a single stress episode.

The mechanisms of immune suppression by high-pressure stress in mice

The effects of high-pressure stress on the induction of anti-sheep red blood cells (SRBC) and of plaque-forming cells (PFC), and on thymus weight, were studied in BALB/c mice in-vivo and in-vitro. The efficacy of high-pressure stress in suppressing PFC and thymic involution was maximum when the stress was applied 1 h day(-1) for 2 days before immunization with SRBC. Both effects were blocked by administration of indomethacin, atropine, naloxone or phentolamine before the first application of stress, whereas hexamethonium and propranolol had no such effect. Hexamethonium, naloxone and propranolol administered before the second application of high-pressure stress blocked both effects. Prostaglandin and acetylcholine given 24 h before application of high-pressure stress caused a marked reduction in PFC count, but not in thymus weight. The reduced PFC count caused by acetylcholine was blocked by pretreatment with indomethacin. When adrenaline was injected 24 h after application of high-pressure stress a marked reduction in PFC was observed, but without thymic involution. When adrenaline was injected 24 h after prostaglandin injection the PFC count was also markedly reduced, but not thymus weight. The decrease in PFC caused by two exposures to stress or one exposure to stress plus injection of adrenaline was blocked by diethylcarbamazine before the second exposure to stress or the injection of adrenaline. In addition, normal spleen cells, were induced as suppressor cells when incubated with the serum of stressed mice, but not when supplemented with anti-leukotriene C4, D4 antibody. These data suggest that mice fall into a pre-stress condition via the release of prostaglandin after the first stress, and then immunosuppression is induced in these prestressed mice via the release of leukotriene C4, D4, caused by the activation of the autonomic nervous system by the second exposure to stress.