Pharmacology of interleukin-1 actions in the brain

Central role of microglia in sepsis-associated encephalopathy: From mechanism to therapy

Sepsis-associated encephalopathy (SAE) is a cognitive impairment associated with sepsis that occurs in the absence of direct infection in the central nervous system or structural brain damage. Microglia are thought to be macrophages of the central nervous system, devouring bits of neuronal cells and dead cells in the brain. They are activated in various ways, and microglia-mediated neuroinflammation is characteristic of central nervous system diseases, including SAE. Here, we systematically described the pathogenesis of SAE and demonstrated that microglia are closely related to the occurrence and development of SAE. Furthermore, we comprehensively discussed the function and phenotype of microglia and summarized their activation mechanism and role in SAE pathogenesis. Finally, this review summarizes recent studies on treating cognitive impairment in SAE by blocking microglial activation and toxic factors produced after activation. We suggest that targeting microglial activation may be a putative treatment for SAE.

Sepsis-associated encephalopathy and septic encephalitis: an update

INTRODUCTION

Sepsis-associated encephalopathy (SAE) and septic encephalitis (SE) are associated with increased mortality, long-term cognitive impairment, and focal neurological deficits.

AREAS COVERED

The PUBMED database was searched 2016-2020. The clinical manifestation of SAE is delirium, SE additionally is characterized by focal neurological symptoms. SAE is caused by inflammation with endothelial/microglial activation, increase of permeability of the blood-brain-barrier, hypoxia, imbalance of neurotransmitters, glial activation, axonal, and neuronal loss. Septic-embolic (SEE) and septic-metastatic encephalitis (SME) are characterized by focal ischemia (SEE) and small abscesses (SME). The continuum between SAE, SME, and SEE is documented by imaging techniques and autopsies. The backbone of treatment is rapid optimum antibiotic therapy. Experimental approaches focus on modulation of inflammation, stabilization of the blood-brain barrier, and restoration of membrane/mitochondrial function.

EXPERT OPINION

The most promising diagnostic approaches are new imaging techniques. The most important measure to fight delirium remains establishment of daily structure and adequate sensory stimuli. Dexmedetomidine and melatonin appear to reduce the frequency of delirium, their efficacy in SAE and SE remains to be established. Drugs already licensed for other indications or available as food supplements which may be effective in SAE are statins, L-DOPA/benserazide, β-hydroxybutyrate, palmitoylethanolamide, and tetracyclines or other bactericidal non-lytic antibiotics.

Cerebrospinal fluid neuron specific enolase, interleukin-1β and erythropoietin concentrations in children after seizures

PURPOSE

In the present study, the levels of neuron-specific enolase (NSE), interleukin-1β (IL-1β), and erythropoietin (EPO) in cerebrospinal fluid (CSF) in children with idiopathic epilepsy were measured to illuminate the relationships between these markers with idiopathic epilepsy.

METHODS

Eighty-five children from 6 months to 12.5 years of age with single, previously undiagnosed, and untreated idiopathic epilepsy were participated in this study. The concentrations of CSF NSE, 1L-1β, and EPO were measured by specific ELISA methods.

RESULTS

The mean concentrations of CSF NSE, IL-1β, and EPO in the epileptic groups showed a significant increase (P < 0.01) compared with those in the control groups. Besides, the mutual correlations of NSE, 1L-1β, and EPO were also analyzed. Results showed that there were positive correlations between the levels of IL-1β, NSE, and EPO.

CONCLUSIONS

The changes of NSE, 1L-1β, and EPO level in CSF may be beneficial for the pathophysiology study of epileptic seizures and the identification and diagnosis of a seizure clinically.

Tumor necrosis factor-alpha expression in peripheral blood mononuclear cells correlates with early childhood social interaction in autism spectrum disorder

Autism spectrum disorder is a neurodevelopmental disorder characterized by impaired social interaction, poor communication skills, and repetitive/restrictive behaviors. Elevated blood levels of pro-inflammatory cytokines have been reported in subjects with autism spectrum disorder. On the other hand, early childhood adverse experience also increases blood levels of these cytokines. Since social experience of children with autism spectrum disorder is generally unlike to typically developing children, we hypothesized that social interaction during childhood contribute to pro-inflammatory cytokine expression in subjects with autism spectrum disorder. We compared revised Autism Diagnostic Interview scores and expression levels of pro-inflammatory cytokines in peripheral blood mononuclear cells of subjects with autism spectrum disorder (n = 30). The score of domain A on the revised Autism Diagnostic Interview, indicating social interaction impairment in early childhood, was negatively correlated with tumor necrosis factor-α mRNA expression level in peripheral blood mononuclear cells but not interleukin-1β or -6. Consistently, tumor necrosis factor-α mRNA expression was markedly low in subjects with autism spectrum disorder compared to typically developing children who presumably experienced the regular levels of social interaction. These findings suggest that the low blood levels of tumor necrosis factor-α mRNA in subjects with autism spectrum disorder might be due to impaired social interaction in early childhood.

Social isolation impairs remyelination in mice through modulation of IL-6

Recent studies have revealed that social experience affects myelination. These findings have important implications for disorders that feature abnormal myelination, such as multiple sclerosis (MS), as previous studies have shown that psychosocial stress exacerbates the pathobiology of MS. However, most studies have focused on psychosocial stress during the demyelination phase of MS and have not investigated the effects of social experience on remyelination. Thus, the current study sought to determine whether social experience can alter remyelination after myelin depletion. Myelin in the mouse medial prefrontal cortex was depleted with cuprizone, and the effects of subsequent social isolation on remyelination were evaluated. Remyelination was severely impaired in socially isolated mice. Social isolation also increased IL-6 levels in the medial prefrontal cortex, and administration of an IL-6 inhibitor (ND₅₀ = 0.01-0.03 μg for 0.25 ng/ml IL-6) ameliorated remyelination impairments. Consistent with this result, IL-6 administration (ED₅₀ = 0.02-0.06 ng/ml) disturbed remyelination. In addition, neuron-oligodendrocyte coculture experiments showed that IL-6 treatment (ED₅₀ ≤ 0.02 ng/ml) markedly impeded myelination, which was recovered with IL-6 inhibitor administration (ND₅₀ = 0.01-0.03 μg for 0.25 ng/ml IL-6). This study provides the first direct evidence, to our knowledge, that social experience influences remyelination via modulation of IL-6 expression. These findings indicate that psychosocial stress may disturb remyelination through regulation of IL-6 expression in patients with such demyelinating diseases that involve remyelination as MS.-Makinodan, M., Ikawa, D., Miyamoto, Y., Yamauchi, J., Yamamuro, K., Yamashita, Y., Toritsuka, M., Kimoto, S., Okumura, K., Yamauchi, T., Fukami, S., Yoshino, H., Wanaka, A., Kishimoto, T. Social isolation impairs remyelination in mice through modulation of IL-6.

Developmental programming of hypothalamic neuroendocrine systems

There is increasing evidence to suggest that the perinatal environment may alter the developmental programming of hypothalamic neuroendocrine systems in a manner that predisposes offspring to the development of metabolic syndrome. Although it is unclear how these effects might be mediated, it has been shown that changes in neuroendocrine programing during critical periods of development, either via maternal metabolic programming or other factors, can alter a fetus's metabolic fate. This review summarizes the hypothalamic circuits that mediate energy homeostasis and discusses the various factors that may influence the development and functioning of these neural systems, as well as the possible cognitive impairments that may arise as a result of these metabolic influences.

Leptin modulates the late fever response to LPS in diet-induced obese animals

Leptin is an important modulator of both inflammation and energy homeostasis, making it a key interface between the inflammatory response to pathogenic stimuli and the energy status of the host. In previous studies we demonstrated that sickness responses to systemic immune challenge, including fever, are significantly exacerbated in diet induced obese animals. To investigate whether this exacerbation is functionally linked to the obesity associated increase in circulating levels of leptin, a species-specific leptin antiserum (LAS) was used to neutralize endogenous leptin in diet-induced obese adult male Wistar rats treated with a single intraperitoneal (i.p.) injection of lipopolysaccharide (LPS) (100μg/kg). LAS significantly reduced the magnitude of the later phases of the fever response, and attenuated the circulating levels of IL-6, IL-1ra and bioactivity of leptin in the obese animals. In addition, the antiserum significantly attenuated the hypothalamic expression of IL-1ß, IκBα, COX2, SOCS3 and IL-6 in both lean and obese rats 10h after the LPS injection and NF-IL6 in the hypothalamus of obese rats only. The relatively late rise in brain IL-6 suggested a role in mediating the extended fever response in obese animals and we tested this by neutralizing brain IL-6 using an IL6-AS injected intracerebroventricularly (4μl, icv). The IL6-AS significantly but transiently (between 9h and 12h post LPS) reduced the late fever response of obese rats. These results demonstrate that leptin plays an important part in modulating the late portion of the fever response to LPS, likely through the induction of hypothalamic IL-6 in obese animals.

Inflammation: a mechanism of depression?

In recent decades, major depression has become more prevalent and research has shown that immune activation and cytokine production may be involved. This review is mainly focused on the contribution of inflammation to depression. We first briefly introduce the inflammatory biomarkers of depression, then discuss the sources of cytokines in the brain, and finally describe the neuroimmunological mechanisms underlying the association between inflammation and depression.

Proinflammatory cytokines in the prefrontal cortex of teenage suicide victims

Teenage suicide is a major public health concern, but its neurobiology is not well understood. Proinflammatory cytokines play an important role in stress and in the pathophysiology of depression-two major risk factors for suicide. Cytokines are increased in the serum of patients with depression and suicidal behavior; however, it is not clear if similar abnormality in cytokines occurs in brains of suicide victims. We therefore measured the gene and protein expression levels of proinflammatory cytokines interleukin (IL)-1β, IL-6, and tissue necrosis factor (TNF)-α in the prefrontal cortex (PFC) of 24 teenage suicide victims and 24 matched normal control subjects. Our results show that the mRNA and protein expression levels of IL-1β, IL-6, and TNF-α were significantly increased in Brodmann area 10 (BA-10) of suicide victims compared with normal control subjects. These results suggest an important role for IL-1β, IL-6, and TNF-α in the pathophysiology of suicidal behavior and that proinflammatory cytokines may be an appropriate target for developing therapeutic agents.

Increased excitotoxicity and neuroinflammatory markers in postmortem frontal cortex from bipolar disorder patients

Reports of cognitive decline, symptom worsening and brain atrophy in bipolar disorder (BD) suggest that the disease progresses over time. The worsening neuropathology may involve excitotoxicity and neuroinflammation. We determined protein and mRNA levels of excitotoxicity and neuroinflammatory markers in postmortem frontal cortex from 10 BD patients and 10 age-matched controls. The brain tissue was matched for age, postmortem interval and pH. The results indicated statistically significant lower protein and mRNA levels of the N-methyl-D-aspartate receptors, NR-1 and NR-3A, but significantly higher protein and mRNA levels of interleukin (IL)-1beta, the IL-1 receptor (IL-1R), myeloid differentiation factor 88, nuclear factor-kappa B subunits, and astroglial and microglial markers (glial fibrillary acidic protein, inducible nitric oxide synthase, c-fos and CD11b) in postmortem frontal cortex from BD compared with control subjects. There was no significant difference in mRNA levels of tumor necrosis factor alpha or neuronal nitric oxide synthase in the same region. These data show the presence of excitotoxicity and neuroinflammation in BD frontal cortex, with particular activation of the IL-R cascade. The changes may account for reported evidence of disease progression in BD and be a target for future therapy.

Leptin modulates cell morphology and cytokine release in microglia

The appetite suppressing hormone, leptin is now established as an important component of the immune response to pathogens partly via the induction of brain IL-1beta. We have previously demonstrated that this hormone acts on microglia to induce the release of IL-1beta through actions on its functional receptors. In the present study, we extended these findings by demonstrating that leptin's action on microglia is that of a modulator rather than a direct trigger of inflammation. Using primary microglia cultures prepared from rat brain we show that pre-incubation of these cells with leptin for 24h prior to treatment with LPS increased the IL-1beta output 2-fold. This effect was not limited to IL-1beta but was also true for another cytokine, TNF-alpha and chemokines such as CINC-1 and MIP-2. The role of leptin in potentiating the microglial response to LPS appeared to be linked to morphological changes rendering the microglia more reactive. These results suggest that leptin has an important role in microglial function in inflammation and given that its circulating levels fluctuate across a number of conditions, these findings can have important implications for an individual's ability to mount an efficient and complete response to invading pathogens.

Mechanisms of brain signaling during sepsis

Brain signaling is a crucial event for the body to mount an appropriate response to invading microorganisms. Pro-inflammatory cytokines are released from infected tissues and reach key structures in the brain via the circumventricular organs, areas of damaged blood brain barrier or they cross actively the blood brain barrier using specific carriers. Alternately, cytokines may activate brain endothelial cells or microglial to produce prostaglandins which then diffuse into the brain to activate neurons. Finally, cytokines may activate the autonomic nervous system at the periphery. The following crosstalk between astrocytes and microglial precedes neuronal activation particularly within the hippocampus, amygdale and hypothalamus. The resulting release of neuro-hormones in the systemic circulation allows restoration of homeostasis. It is likely that an excess in nitric oxide and complement anaphylatoxin C5a contributes to DNA damage within neurons of the hippocampus and hypothalamus and subsequent brain dysfunction.

Regulation of agouti-related protein messenger ribonucleic acid transcription and peptide secretion by acute and chronic inflammation

Agouti-related protein (AgRP) is an orexigenic neuropeptide produced by neurons in the hypothalamic arcuate nucleus (ARC) that is a key component of central neural circuits that control food intake and energy expenditure. Disorders in energy homeostasis, characterized by hypophagia and increased metabolic rate, frequently develop in animals with either acute or chronic diseases. Recently, studies have demonstrated that proopiomelanocortin-expressing neurons in the ARC are activated by the proinflammatory cytokine IL-1beta. In the current study, we sought to determine whether inflammatory processes regulate the expression of AgRP mRNA and to characterize the response of AgRP neurons to IL-1beta. Here, we show by real-time RT-PCR and in situ hybridization analysis that AgRP mRNA expression in rodents is increased in models of acute and chronic inflammation. AgRP neurons were found to express the type I IL-1 receptor, and the percentage of expression was significantly increased after peripheral administration of lipopolysaccharide. Furthermore, we demonstrate that IL-1beta inhibits the release of AgRP from hypothalamic explants. Collectively, these data indicate that proinflammatory signals decrease the secretion of AgRP while increasing the transcription of the AgRP gene. These observations suggest that AgRP neurons may participate with ARC proopiomelanocortin neurons in mediating the anorexic and metabolic responses to acute and chronic disease processes.

Regulation of central melanocortin signaling by interleukin-1 beta

Anorexia and involuntary weight loss are common and debilitating complications of a number of chronic diseases and inflammatory states. Proinflammatory cytokines, including IL-1 beta, are hypothesized to mediate these responses through direct actions on the central nervous system. However, the neural circuits through which proinflammatory cytokines regulate food intake and energy balance remain to be characterized. Here we report that IL-1 beta activates the central melanocortin system, a key neuronal circuit in the regulation of energy homeostasis. Proopiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus (ARC) were found to express the type I IL-1 receptor. Intracerebroventricular injection of IL-1 beta induced the expression of Fos protein in ARC POMC neurons but not in POMC neurons in the commissural nucleus of the tractus solitarius. We further show that IL-1 beta increases the frequency of action potentials of ARC POMC neurons and stimulates the release of alpha-MSH from hypothalamic explants in a dose-dependent fashion. Collectively, our data support a model in which IL-1 beta increases central melanocortin signaling by activating a subpopulation of hypothalamic POMC neurons and stimulating their release of alpha-MSH.

Glial-cytokine-neuronal interactions underlying the mechanisms of persistent pain

The emerging literature implicates a role for glia/cytokines in persistent pain. However, the mechanisms by which these non-neural elements contribute to CNS activity-dependent plasticity and pain are unclear. Using a trigeminal model of inflammatory hyperalgesia, here we provide evidence that demonstrates a mechanism by which glia interact with neurons, leading to activity-dependent plasticity and hyperalgesia. In response to masseter inflammation, there was an upregulation of glial fibrillary acidic proteins (GFAPs), a marker of astroglia, and interleukin-1beta (IL-1beta), a prototype proinflammatory cytokine, in the region of the trigeminal nucleus specifically related to the processing of deep orofacial input. The activated astroglia exhibited hypertrophy and an increased level of connexin 43, an astroglial gap junction protein. The upregulated IL-1beta was selectively localized to astrocytes but not to microglia and neurons. Local anesthesia of the masseter nerve prevented the increase in GFAP and IL-1beta after inflammation, and substance P, a prototype neurotransmitter of primary afferents, induced similar increases in GFAP and IL-1beta, which was blocked by a nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester. Injection of IL-1 receptor antagonist and fluorocitrate, a glial inhibitor, attenuated hyperalgesia and NMDA receptor phosphorylation after inflammation. In vitro application of IL-1beta induced NR1 phosphorylation, which was blocked by an IL-1 receptor antagonist, a PKC inhibitor (chelerythrine), an IP3 receptor inhibitor (2-aminoethoxydiphenylborate), and inhibitors of phospholipase C [1-[6-((17b-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-1H-pyrrole-2,5-dione] and phospholipase A2 (arachidonyltrifluoromethyl ketone). These findings provide evidence of astroglial activation by tissue injury, concomitant IL-1beta induction, and the coupling of NMDA receptor phosphorylation through IL-1 receptor signaling.

Neuropeptide Y: potential role in recurrent developmental seizures

Seizures induce profound plastic changes in the brain, including altered expression of neuropeptide Y (NPY) and its receptors. Here, I discuss a potential role of NPY plasticity in the developmental brain: in a rat model of febrile seizures (FS), the most common type of seizures in infants and young children, NPY expression was up-regulated in hippocampus after experimentally induced FS. Interestingly, NPY up-regulation was associated with an increased seizure threshold for additional (recurrent) FS, and this effect was abolished when an antagonist against NPY receptor type 2 was applied. These findings suggest that inhibitory actions of NPY, released after seizures, exert a protective effect that reduces the risk of seizure recurrence in the developing brain.

Age-related changes in neuroprotection: is estrogen pro-inflammatory for the reproductive senescent brain?

Estrogen replacement therapy (ERT) is widely prescribed to postmenopausal women for relief from the adverse vasomotor effects of menopause, to reduce bone loss, to improve cardiovascular health, and to protect against metabolic disorders. However, there is now greater awareness of the increased risk to benefit ratio from the recently concluded Women's Health Initiative Memory Study (WHIMS), which reported that ERT increased the risk of cognitive impairment and dementia in elderly women. Studies from the experimental literature indicate that while estrogen is neuroprotective in many instances, estrogen replacement can be deleterious in some cases. These differences may be partly due to the age and species of the experimental model. The majority of the experimental data comes from studies where the age or endocrine status of the animal model is not comparable to that of menopausal or postmenopausal women, such as those in the WHIMS study. In this review, we will focus on age-related changes in estrogen's neuroprotective effects and evidence that reproductive senescence-related changes in the blood-brain barrier and the immune system may result in deleterious consequences for ERT.

Modulatory effect of interleukin-1β on rat isolated basilar artery contraction

An increased level of cytokine interleukin-1 (IL-1) has been detected around the site of stroke. However, the effect of IL-1beta on the basilar artery has received little attention. We evaluated the effects of IL-1beta on the contractile response of rat isolated basilar artery by measuring isometric tension change. IL-1beta (10 ng/ml) and phenylephrine (0.1 nM) markedly enhanced U46619 (30 and 100 nM)-induced basilar artery contraction. The IL-1beta-mediated potentiation was partly suppressed by zinc protoporphyrin (3 microM) and was abolished by tetrodotoxin (TTX, 100 nM), (-)-perillic acid (1 microM), PD98059 (0.3 microM), SB203580 (1 microM) and prazosin (1 microM). Our data suggest that IL-1beta (10 ng/ml) causes an enhancement of U46619-mediated basilar artery contraction that probably involves TTX-sensitive neuronal release of an alpha1-adrenoceptor agonist and activation of p42/p44 and p38 mitogen-activated protein kinases/p21(ras) pathways.

Microglial cell population dynamics in the injured adult central nervous system

Reactive microgliosis is characteristic of trauma and stroke as well as inflammatory and chronic neurodegenerative disease. A conspicuous feature of the microglial reaction to acute neural injury is a massive expansion of the microglial cell population which peaks a few days following injury. New data based on the use of radiation bone marrow-chimeric mice suggest this expansion also involves recruitment of bone marrow-derived cells, which migrate into the neural parenchyma and differentiate into microglia. Here, we discuss the contribution of bone marrow-derived cells to the injury-induced expansion of the microglial cell population, seen in the dentate gyrus with ongoing anterograde axonal and terminal synaptic degeneration, subsequent to transection of the entorhino-dentate perforant path projection. In this paradigm of minor brain injury, the bone marrow-derived cells are grossly outnumbered by activated resident microglia, which express the stem cell antigen CD34 concurrent to a marked capacity for self-renewal. The observation of a mixed origin of lesion-reactive microglia, consisting of a smaller subpopulation of exogenous bone marrow-derived microglia, and a larger population of activated resident microglia, the majority of which express CD34 and undergo proliferation, suggests that lesion-reactive microglia consist of functionally distinct cell populations. The demonstration of an injury-enhanced recruitment of bone marrow-derived cells into the perforant path-denervated dentate gyrus, raises the possibility of using genetically manipulated cells as vectors for lesion-site-specific gene therapy even in minimally injured areas of the central nervous system.

Learning modulation by endogenous hippocampal IL-1: Blockade of endogenous IL-1 facilitates memory formation

The interleukin-1 (IL-1) cytokine family (IL-1alpha, IL-beta, and the IL-1 receptor antagonist) is involved in immune and inflammatory responses both in the brain and in the periphery. Recently, it has also been shown to influence behavior and memory consolidation. However, within the experimental systems studied, it has remained unclear whether the role of IL-1beta is associated solely with a pathophysiological process or whether it is a neuromodulator in normal adult brain. To evaluate the involvement of the nonpathological endogenous IL-1 system in learning, we studied the expression of IL-1alpha, IL-1beta, and IL-1ra during memory consolidation. We observed a learning-specific hippocampal IL-1alpha mRNA induction, but not that of IL-1beta or IL-1ra mRNAs, after inhibitory avoidance training. Moreover, when IL-1 receptor activity was inhibited using an adenoviral vector that expresses the IL-1 receptor antagonist (IL-1ra) in the hippocampus, both short-term and long-term memory retention scores were facilitated. In contrast, endogenous hippocampal IL-1 played no role in the habituation to a novel environment. These results demonstrate that endogenous hippocampal IL-1 specifically modulates a fear-motivated learning task, and suggest that IL-1alpha activity in the CNS is part of the hippocampal memory processing.

Effect of central administration of interleukin-1 receptor antagonist on protein synthesis in skeletal muscle, kidney, and liver during sepsis

Inflammatory cytokines may mediate the host response to infection via central nervous system (CNS), endocrine, and/or paracrine pathways. The purpose of the present study was to determine whether intracerebroventricular (ICV) infusion of interleukin-1 receptor antagonist (IL-1ra) influences the effects of sepsis on protein metabolism in peripheral organs (skeletal muscle, kidney, and liver). A constant ICV infusion of IL-1ra (100 microg/h) or saline was begun immediately before the induction of sepsis or sterile inflammation and continued for 5 days. ICV infusion of IL-1ra did not alter protein metabolism in animals with a sterile abscess. Sepsis reduced muscle weight, protein content, and rates of protein synthesis in gastrocnemius. ICV infusion of IL-1ra attenuated the sepsis-induced loss of muscle mass and protein and the inhibition of protein synthesis in gastrocnemius by augmenting the translational efficiency. Similar results were observed in kidney, with respect to kidney weight, total protein, rates of protein synthesis, and translational efficiency. However, central infusion of IL-1ra did result in a small (12%) increase in the renal RNA content in either sterile or septic abscess rats. In liver, ICV infusion of IL-1ra prevented the sepsis-induced inhibition of protein synthesis and reduction in translational efficiency. These results suggest that central administration IL-1ra can modulate protein metabolism in peripheral organs during sepsis by preventing the sepsis-induced defects in the translational efficiency.

Prior stressor exposure sensitizes LPS-induced cytokine production

Exposure to stressors often alters the subsequent responsiveness of many systems. The present study tested whether prior exposure to inescapable tailshock (IS) alters the interleukin (IL)-1beta, tumor necrosis factor (TNF)-alpha, or IL-6 response to an injection of bacterial endotoxin (lipopolysaccharide; LPS). Rats were exposed to IS or remained as home cage controls (HCC); 24 h later animals were injected i.p. with either 10 microg/kg LPS or equilvolume sterile saline. IS significantly increased plasma TNF-alpha, IL-1beta, and pituitary, hypothalamus, hippocampus, cerebellum IL-1beta 1 h, but not 2 h, after LPS, compared to controls. Additional animals were injected with LPS or saline 4, 10, or 21 days after exposure to IS and tail vein blood was collected and assayed for IL-1beta. An enhanced plasma IL-1beta response occurred 4 days after IS, but was gone by 10 days. These results suggest that exposure to IS sensitizes the innate immune response to LPS by resulting in either a larger or a more rapid induction of proinflammatory cytokines.

Anorexic but not pyrogenic actions of interleukin-1 are modulated by central melanocortin-3/4 receptors in the rat

The cytokine interleukin-1 (IL-1), which mediates many responses to infection and injury, induces anorexia and fever through direct actions in the central nervous system. The melanocortin neuropeptides, such as alpha melanocyte-stimulating hormone (alpha-MSH), reportedly antagonize many actions of IL-1, including fever and anorexia. However, it is unknown whether endogenous melanocortins modulate anorexia induced by IL-1. The objective of the present study was to establish the effect of endogenous melanocortins on IL-1-induced anorexia and fever in the rat. Intracerebroventricular (i.c.v.) injection of IL-1beta caused a significant reduction in food intake and body weight gain, and a rise in core body temperature in conscious rats. Coadministration of the melanocortin-3/4 receptor (MC3/4-R) antagonist, SHU9119, reversed IL-1beta-induced reductions in food intake and body weight, but did not affect the febrile response to IL-1beta. These data suggest IL-1beta may elicit its effects on food intake through the melanocortin system, predominantly via the MC3-R or MC4-R. In contrast, IL-1beta-induced fever does not appear to be mediated or modulated by MC3-R or MC4-R activity.

The role of pro- and antiinflammatory cytokines in neurodegeneration

Experimental and clinical damage to the brain leads to rapid upregulation of an array of cytokines predominantly by glia. These cytokines may exert neurotoxic or neuroprotective actions. This paper will focus on the pro-inflammatory cytokine interleukin-1 (IL-1), which participates in diverse forms of brain damage including ischemia, brain trauma, and excitotoxic injury. Administration of low doses of IL-1 markedly exacerbates these forms of brain damage, whereas blocking IL-1 release or actions reduces neuronal death. IL-1 receptor antagonist (IL-1ra) is also upregulated by brain damage (mainly by neurons) and acts as an endogenous inhibitor of neurodegeneration, presumably by blocking IL-1 actions on its receptor. We have studied the actions of both IL-1 and IL-1ra in experimental models of ischemic and neurotoxic injury in rats, and have found site-specific effects within the striatum. On the basis of this and further work, we propose that IL-1 can exacerbate cell death in these conditions by modifying polysynaptic anterograde pathways leading from the striatum to the cortex. The precise nature of these pathways remains undetermined, as do the underlying mechanisms by which IL-1 can exert its effects, but appear to involve induction of IL-1 in specific brain regions and activation of cortical glutamatergic pathways.

Intrathecally administered endotoxin or cytokines produce allodynia, hyperalgesia and changes in spinal cord neuronal responses to nociceptive stimuli in the rat

Inflammatory processes occurring within the central nervous system (CNS) can produce 'illness induced behaviours' which include fever, sleep and the development of allodynia and hyperalgesia. Here we demonstrate the effects of the pro-inflammatory mediators, bacterial endotoxin, and rat recombinant interleukin 1 beta (rrIL-1 beta) or tumour necrosis factor-alpha (rrTNF alpha) on the integration of somatosensory information at the single neuronal level, via recordings from wide-dynamic range neurones in the dorsal horn of the spinal cord in anaesthetized rats. Intrathecal administration of E. coli lipopolysaccharide (LPS, 10 and 100 microg, i.t.) enhanced the activity of dorsal horn neurones, including facilitation of neuronal post-discharge. Intrathecal administration of IL-1 beta (5-5000 pg) or TNF-alpha (5-5000 pg) enhanced dorsal horn neuronal responses, including the acute responses to C-fibre stimulation, wind-up and post-discharge, however, the effects of IL-1 beta were more robust than those of TNF alpha. Intrathecal administration of IL-1 beta (1-1000 pg) also leads to the development of mechanical allodynia and hyperalgesia. On the other hand intrathecal application of TNF alpha did not produce changes in sensitivity to mechanical stimuli. Changes in the activity of spinal wide-dynamic range neurones induced by local inflammation may provide a pathomechanism for the clinical pathology of central pain syndrome, which can accompany CNS disease or acute CNS injury.

The pain of being sick: implications of immune-to-brain communication for understanding pain

This review focuses on the powerful pain facilitatory effects produced by the immune system. Immune cells, activated in response to infection, inflammation, or trauma, release proteins called proinflammatory cytokines. These proinflammatory cytokines signal the central nervous system, thereby creating exaggerated pain as well as an entire constellation of physiological, behavioral, and hormonal changes. These changes are collectively referred to as the sickness response. Release of proinflammatory cytokines by immune cells in the body leads, in turn, to release of proinflammatory cytokines by glia within the brain and spinal cord. Evidence is reviewed supporting the idea that proinflammatory cytokines exert powerful pain facilitatory effects following their release in the body, in the brain, and in the spinal cord. Such exaggerated pain states naturally occur in situations involving infection, inflammation, or trauma of the skin, of peripheral nerves, and of the central nervous system itself. Implications for human pain conditions are discussed.

Nitric oxide regulates body temperature, neuronal activation and interleukin-1 beta gene expression in the hypothalamic paraventricular nucleus in response to immune stress

An immune challenge initiates a complex cascade of events in the body including important responses from the central nervous system. As nitric oxide (NO) has been implicated in the central regulation of neuroendocrine and autonomic responses, this study was performed to determine if NO regulates physiological responses, neuronal activation, and/or interleukin-1 beta (IL-1 beta) gene expression in the paraventricular nucleus of the rat hypothalamus (PVN) in response to intravenous endotoxin, lipopolysaccharide (LPS, 100 microg/kg). Intracerebroventricular injections of NO synthase (NOS) inhibitors (7-nitroindazole sodium salt for neuronal NOS, N(G)-nitro-L-arginine for neuronal NOS and endothelial NOS, and aminoguanidine for inducible NOS) in LPS-treated rats showed that inhibition of NOS eliminated the drop in body temperature and led to increased neuronal activation in the PVN as assessed by immunohistochemistry for Fos-like immunoreactivity. Activation of NO-producing PVN neurons was also increased in these rats suggesting that NO influences neuronal NOS activity in PVN neurons. Finally, increased IL-1 beta gene expression in the PVN of LPS-treated rats receiving N(G)-nitro-L-arginine showed that NO regulates brain IL-1 beta gene expression. The results obtained with the NOS inhibitors support the hypothesis that NO produced from eNOS in the brain participates in temperature regulation, and inhibits PVN neuronal activity and IL-1 beta gene expression during immune stress.

Brain interleukin-1beta and tumor necrosis factor-alpha are involved in lipopolysaccharide-induced delayed rectal allodynia in awake rats

Recently, we have developed a model of delayed (12 h) increase in sensitivity (allodynia) to rectal distension (RD) induced by intraperitoneal lipopolysaccharide (LPS) in awake rats. Thus, we examined whether central interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha) are involved in LPS response. Abdominal contractions (criterion of visceral pain) were recorded in rats equipped with intramuscular electrodes. RDs were performed at various times after pharmacological treatments. RD induced abdominal contractions from a threshold volume of distension of 0.8 ml. At lowest volume (0.4 ml), this number was significantly increased 12 h after LPS. Intracerebroventricular (i.c.v.) injection of IL-1 receptor antagonist, IL-1beta converting enzyme inhibitor or recombinant human TNF-alpha soluble receptor reduced LPS-induced increase of abdominal contractions at 0.4 ml volume of distension. When injected i.c.v., recombinant human IL-1beta and recombinant bovine TNF-alpha reproduced LPS response at 9 and 12 h and at 6 and 9 h, respectively. These data suggest that IL-1beta and TNF-alpha act centrally to induce delayed rectal hypersensitivity and that central release of these cytokines is responsible of LPS-induced delayed (12 h) rectal allodynia.

Dynamic regulation of the proinflammatory cytokine, interleukin-1beta: molecular biology for non-molecular biologists

Interleukin-1beta (IL-1beta) is a key mediator and modulator of a wide array of physiological responses important for survival. It is created by a variety of cell types, including immune cells, glia, and neurons. It is a very potent biological molecule, acting both at the periphery as well as within the central nervous system. The production and release of IL-1beta is tightly regulated by far more complex processes than previously thought. An appreciation of this complexity is necessary for proper interpretation of apparent contradictions in the literature where different aspects of IL-1beta expression are measured. Given that many researchers are not molecular biologists by training, yet need an appreciation of the controls that regulate the function of key proteins such as IL-1beta, this review is aimed at both: (a) clarifying the multiple levels at which IL-1beta production is modulated and (b) using IL-1beta regulation to explain the dynamics of gene regulation to non-molecular biologists. Three major topics will be discussed. First, regulation of IL-1beta production will be examined at every level from extracellular signals that trigger gene activation through release of active protein into the extracellular fluid. Second, regulation of IL-1beta bioavailability and bioactivity will be discussed. This section examines the fact that even after IL-1beta is released, it may or may not be able to exert a biological action due to multiple modulatory factors. Last is the introduction of the idea that IL-1beta regulation is, at times, beyond the direct control of host; that is, when IL-1beta production becomes dysregulated by pathogens.

IL-1beta and IL-6 excite neurons and suppress nicotinic and noradrenergic neurotransmission in guinea pig enteric nervous system

Conventional intracellular microelectrodes and injection of biocytin were used to study the actions of IL-1beta and IL-6 on electrical and synaptic behavior in morphologically identified guinea pig small intestinal submucous neurons. Exposure to nanomolar concentrations of either IL-1beta or IL-6 stimulated neuronal excitability. The excitatory action consisted of depolarization of the membrane potential, decreased membrane conductance, and increased discharge of action potentials. Excitatory action of IL-1beta was suppressed by the natural IL-1beta human receptor antagonist. Electrical stimulation of sympathetic postganglionic axons evoked inhibitory postsynaptic potentials (IPSPs), and stimulation of cholinergic axons evoked nicotinic fast excitatory postsynaptic potentials (EPSPs). Both kinds of synaptic potentials occurred in neurons with uniaxonal morphology believed to be secretomotor neurons. Either IL-1beta or IL-6 suppressed the noradrenergic IPSPs and the fast EPSPs, and the two acted synergistically when applied in combination. Suppression of the IPSP resulted from presynaptic inhibition of the release of norepinephrine from sympathetic nerves. The results suggest that the presence of either or both inflammatory cytokines will release the sympathetic brake from secretomotor neurons to the intestinal crypts and from nicotinic synapses in the integrative microcircuits, where norepinephrine is known to have a presynaptic inhibitory action. This, in concert with excitation of secretomotor neurons, may lead to neurogenic secretory diarrhea.

Induction of pro-inflammatory cytokine mRNAs in the brain after peripheral injection of subseptic doses of lipopolysaccharide in the rat

Although it is generally accepted that pro-inflammatory cytokines produced by cells of the central nervous system play important roles in the communication between the central nervous system and the immune system during sepsis, it is not clear whether these cytokines are produced in the brain under subseptic conditions. In this study, we used in situ hybridization to examine the mRNA expression of the pro-inflammatory cytokines IL-1beta and TNFalpha in the brains of rats 2 and 12 h after they were challenged by peripheral injections of lipopolysaccharide (LPS) ranging from 0.01 to 1000 microg/kg. Unlike septic doses of LPS (> 500 microg/kg), which induce global expression of pro-inflammatory cytokines in the brain, subseptic doses of LPS (0.01-10 microg/kg) induced IL-1beta and TNFalpha mRNA expression only in the choroid plexus, the circumventricular organs, and meninges. The expression of the cytokine-responsive immediate early gene I kappaB alpha was induced in the brain after doses of LPS as low as 0.1 microg/kg. I kappaB alpha mRNA expression was confined to sites where IL-1beta and TNFalpha were expressed. These results indicate that the induction and action of pro-inflammatory cytokines during subseptic infection occur at the blood-brain barrier and at circumventricular organs, which may be sites for elaboration of signal molecules that communicate peripheral immune status to the brain.

Annual review prize lecture cytokines - killers in the brain?

Given at the Meeting of the Physiological Society held at the University of Southampton on 10 September 1998

Interleukin-1 and nociception in the rat

It has recently become accepted that several cytokines may affect peripheral and central nervous system functions. Consistently with these findings, accumulating evidence points toward an important role for interleukin- in the modulation of nociceptive information. Here we review the observations collected after the administration of this cytokine by intracerebroventricular, intrathecal or peripheral route in rats. Taken together, these data suggest that IL-1 can differently affect pain responsivity depending on the dose and the site of action, and clearly demonstrate that this immune factor is deeply involved in the modulation of neuronal functions.

The role of the vagus nerve in cytokine-to-brain communication

Peripheral interleukin-1 beta (IL-beta) and inflammatory stimuli that induce the synthesis and release of IL-1 beta produce a variety of central nervous system responses. Most proposals designed to explain how peripheral IL-1 beta influences the CNS have focused on blood-borne routes of communication. We will review data that indicate that at least some of the CNS response to peripheral IL-1 beta are instead mediated by a neural route of communication between the periphery and the CNS. IL-1 beta activates afferent vagal fibers that terminate in the nucleus tractus solitarius, and communication via the vagus is responsible for much of the hyperalgesia, fever, anorexia, taste aversions, increased levels of plasma corticosteroid, and brain norepinephrine changes produced by intraperitoneal injections of IL-1 beta and LPS. Data extending this analysis to TNF-alpha and intravenous routes will be described.

Role of central IL-1 in regulating peripheral IGF-I during endotoxemia and sepsis

Inflammatory cytokines may mediate the host response to infection via central nervous system, endocrine, and/or paracrine/autocrine signaling mechanisms. Previous studies have shown that intravenous administration of interleukin (IL)-1 beta alters the concentration of the anabolic hormone insulin-like growth factor (IGF)-I in plasma and various tissues. The purpose of the present study was to determine 1) whether the intracerebroventricular injection of IL-1 beta can influence peripheral IGF-I levels in control animals and 2) whether the central administration of a IL-1 receptor antagonist (IL-1ra) can prevent the changes in peripheral IGF-I induced by endotoxin [lipopolysaccharide (LPS)] or sepsis produced by cecal ligation and puncture. In the first experiment, injection of IL-1 beta (100 ng/rat) decreased IGF-I levels in plasma, liver, and gastrocnemius muscle 28-36% by 1.5 h in conscious fasted rats. IGF-I levels remained reduced at 3 h, but returned to baseline by 6 h. IGF-I content was not altered in soleus, kidney, spleen, intestine, or whole brain after IL-1 beta. In the second series of experiments, LPS injected intravenously decreased IGF-I levels in plasma, liver, and gastrocnemius at 1.5 h, and levels were even further reduced at 3 and 6 h in these tissues (59, 57, and 48%, respectively). Moreover, the IGF-I content was also decreased in soleus (30-35%) and increased in kidney (2- to 3-fold) after LPS. In the third experiment, changes in IGF-I levels in plasma and tissues, similar to those seen in LPS-treated rats, were detected 24 h after induction of peritonitis. Intracerebroventricular infusion of IL-1ra did not alter any of the changes in IGF-I produced by either LPS or sepsis, although it did attenuate the concomitant changes in growth hormone levels. These data suggest that, although central IL-1 beta is capable of modulating peripheral IGF-I levels, central administration of IL-1ra was unable to modulate the changes in peripheral IGF-I in blood and tissues produced by either endotoxemia or peritonitis.

The fatty acid composition of maternal diet affects the response to excitotoxic neural injury in neonatal rat pups

Fatty acids and their derivatives play a role in the response to neural injury. The effects of prenatal and postnatal dietary fatty acid composition on excitotoxic neural injury were investigated in neonatal rat pups. Dams were fed during gestation and lactation a diet whose fat source was either corn oil or menhaden fish oil. On postnatal day 3, litters were culled to 10 per dam. On postnatal day 4, excitotoxic neural injury was induced by infusion of the glutamate analog N-methyl-DL-aspartate (NMA) into the left cerebral hemisphere. Three days later, pups were killed and brains were removed for histological and volume assessments. Levels of arachidonic acid were 2.3-fold higher in cerebrums of pups in the corn oil group than in the fish oil group. Left cerebral hemispheres among all pups were atrophic. Right cerebral hemispheres of pups in the corn oil group showed more histological evidence of edema, and had significantly higher volumes than pups in the fish oil group (66 vs. 42 mm2, p=0.007). These data suggest that the fatty acid composition of prenatal and/or postnatal diet can affect the neonatal response to excitotoxic neural injury.

Time course and localization patterns of interleukin-1β messenger rna expression in brain and pituitary after peripheral administration of lipopolysaccharide

The inflammatory cytokine interleukin-1 has been implicated as a mediator of many centrally controlled responses, such as fever and increased activity of the hypothalamic-pituitary adrenal axis, after systemic infections. To identify the neuroanatomical loci of brain interleukin-1-producing cells during infection, we investigated interleukin-1beta messenger RNA expression by in situ hybridization histochemistry using a 500 nt ribonucleotide probe applied on brain sections from rats injected intraperitoneally with 2.5 mg/kg bacterial lipopolysaccharide or saline. In control animals, interleukin-1beta messenger RNA was not detectable. In the brains of lipopolysaccharide-injected animals, two temporally and spatially distinct waves of interleukin-1beta messenger RNA induction were observed. First, cell labelling appeared at 0.5 h, peaked at 2 h, and declined at 4-8 h. The labelled cells were concentrated in circumventricular organs--organum vasculosum of the lamina terminalis, subfornical organ, median eminence, and area postrema--and in choroid plexus, meninges, and blood vessels. Second, at 8-12 h, scattered small cells became labelled throughout the entire brain parenchyma; the labelling subsided by 24 h. Labelling was not observed in any neurons. In the pituitary, lipopolysaccharide induced strong interleukin-1beta messenger RNA expression initially in the anterior lobe at 0.5-1 h, and later in the neural lobe at 1-2 h, and subsiding thereafter. The results show that at early time points, peripheral lipopolysaccharide induces interleukin-1beta message production at the blood brain barrier and in circumventricular organs where the blood brain barrier is leaky. After a time delay of 6-10 h, however, interleukin-1beta messenger RNA is primarily expressed by non-neuronal cells of the brain in the brain parenchyma. These results suggest that the source of initial brain IL-1 activity after peripheral lipopolysaccharide injection derives from cells of the blood-brain barrier and the circumventricular organs, and the sustained interleukin-1 activity in the central nervous system thereafter is derived from glia.

Hypoxic-ischemic injury induces monocyte chemoattractant protein-1 expression in neonatal rat brain

Monocyte chemoattractant protein-1 (MCP-1) regulates monocyte accumulation in several macrophage-dependent experimental disease models. In the neonatal brain, activated microglia accumulate rapidly after hypoxic-ischemic injury. These cells produce potentially neurotoxic factors that may contribute to the progression of injury. To determine whether MCP-1 could be one of the molecular signals that influences the microglial response to hypoxic-ischemic injury in the neonatal brain, we examined the impact of acute hypoxic-ischemic injury on MCP-1 mRNA and protein expression. Seven-day-old rats underwent right carotid artery ligation, followed by 3 hours of 8% oxygen exposure, to elicit ipsilateral forebrain hypoxic-ischemic injury. To detect MCP-1 mRNA in situ hybridization assays were performed using 35S-labeled antisense riboprobes generated from rat MCP-1 cDNA. Animals were evaluated 0, 1, 2, 4, 8, 16, 24, 48, and 120 hours after hypoxic exposure (N > or = 3/group). Immunocytochemistry (with a polyclonal rabbit antirat MCP-1 antibody) was used to determine the anatomic and temporal distribution of MCP-1, in samples obtained 10 minutes to 5 days after hypoxic exposure (N > or = 3/group). Monocyte chemoattractant protein-1 mRNA was first detected in periventricular regions of the lesioned hemisphere 1 hour after hypoxia-ischemia; periependymal and intraparenchymal MCP-1 mRNA expression were detected at 4 hours; hybridization signal peaked at 8 to 24 hours; and no MCP-1 mRNA was detected at 48 and 120 hours. In lesioned forebrain, MCP-1 protein expression were consistently detected at 2.5 to 48 hours after hypoxia-ischemia. Many immunoreactive cells appeared to be neurons. These results suggest that in the developing brain, MCP-1 could represent a functionally important molecular signal for the microglial response to hypoxic-ischemic injury.

Overexpression of interleukin-1 receptor antagonist in the mouse brain reduces ischemic brain injury

It has been reported that middle cerebral artery occlusion in rats causes overexpression of interleukin-1, and that administration of the interleukin-1 receptor antagonist protein (IL-1ra) reduces ischemic brain injury. The aim of the present study is to determine whether a recombinant adenovirus vector carrying human interleukin-1 receptor antagonist cDNA (Ad.RSVIL-1ra) could be used to overexpress IL-1ra in mouse brain and to evaluate its effect on brain edema formation and infarction after permanent focal ischemia in mice. Ad.RSVIL-1ra, control adenovirus containing the lacZ gene (Ad.RSVlacZ), or saline was injected into the right cerebral ventricle in mice. Brain IL-1ra concentrations were measured 1 to 13 days later. On the fifth day after virus injection, the middle cerebral artery was occluded for 24 h. Brain water content was determined and a histological technique was used to measure the infarction size. Overexpression of human IL-1ra protein in whole brain was confirmed by immunoassay in the Ad.RSVIL-1ra injected mice. It began on the first day, peaked at 5-7 days, and was sustained for 13 days. Brain edema and cerebral infarct volume were significantly reduced following 24 h of permanent middle cerebral artery occlusion in mice transfected with Ad.RSVIL-1ra compared to Ad.RSVlacZ or saline 5 days earlier. These studies demonstrate that adenoviral vectors can be used to deliver genes to small animals such as mice and also suggest the feasibility of gene therapy for stroke and other neurological diseases. Overexpression of human IL-1ra attenuated ischemic brain injury, suggesting that IL-1 may play an important role in cerebral ischemia.

Interleukin-1 and defensins in thermoregulation, stress, and immunity

Our previous studies suggested that IL-1 and defensins, which play a critical role in mechanisms of host resistance, participated in the realization of stress reaction. The present report describes functional relations between IL-1 and defensins during stress reactions and the influence of defensins administration on thermoregulation and the IL-1 level in blood. It is shown that stress-induced shifts in the humoral immune response are accompanied by diverse changes in the IL-1 alpha level in blood and LAF production by peritoneal macrophages, which are possibly associated with the intensity and strength of applied stressors. Functional interrelations between actions of defensins and IL-1 during stress reaction and in the conditions of an LPS-induced rise in body temperature was demonstrated. These results suggest that antibacterial peptides defensins may be involved in thermoregulation in case of increased body temperature probably influencing the blood level of IL-1.

Immune activation: the role of pro-inflammatory cytokines in inflammation, illness responses and pathological pain states

It has recently become accepted that the activated immune system communicates to brain via release of pro-inflammatory cytokines. This review examines the possibility that pro-inflammatory cytokines (interleukins and/or tumor necrosis factor) mediate a variety of commonly studied hyperalgesic states. We will first briefly review basic immune responses and inflammation. We will then develop the concept of illness responses and provide evidence for their existence and for the dramatic changes in neural functioning that they cause. Lastly, we will examine the potential roles that both pro-inflammatory cytokines and the neural circuits that they activate may play in the hyperalgesic states produced by irritants, inflammatory agents, and nerve damage. The possibility is raised that apparently diverse hyperalgesic states may converge in the central nervous system and activate similar or identical neural circuitry.

Mechanisms of tumor necrosis factor-alpha (TNF-alpha) hyperalgesia

Activation of immune cells by pathogens induces the release of a variety of proinflammatory cytokines, including IL-1 beta and TNF-alpha. Previous studies using IL-1 beta have demonstrated that this cytokine can alter brain function, resulting in a variety of 'illness responses' including increased sleep, decreased food intake, fever, etc. We have recently demonstrated that i.p. IL-1 beta also produces hyperalgesia and that this hyperalgesia (as well as most illness responses) is mediated via activation of subdiaphragmatic vagal afferents. The present series of studies were designed to provide an initial examination of the generality of proinflammatory cytokine-induced hyperalgesia by examining the effects of i.p. TNF-alpha on pain responsivity. These studies demonstrate that: (a) i.p. TNF-alpha produces dose-dependent hyperalgesia as measured by the tailflick test, (b) this hyperalgesia is mediated via the induced release of IL-1 beta, (c) hyperalgesia is mediated via activation of subdiaphragmatic vagal afferents, and (d) the effects of subdiaphragmatic vagotomy cannot be explained by a generalized depression of neural excitability.

Cytokine-to-brain communication: a review & analysis of alternative mechanisms

It is becoming well accepted that products of the immune system (cytokines) can signal the brain that infection has occurred. This cytokine-to-brain communication can result in marked alterations in brain function and behavior. This review examines alternative mechanisms that have been proposed to explain how such immune products can reach the brain via the blood to cause centrally-mediated "illness" responses. Finally, we describe a new view which argues that cytokines signal brain in quite a different manner, by stimulating afferent terminals of peripheral nerves at local sites of synthesis and release.

Interleukin-1 alpha and vasoactive intestinal peptide: enigmatic regulation of neuronal survival

A neurotrophic role for interleukin-1 alpha (IL-1 alpha) was investigated in dissociated spinal cord-dorsal root ganglion cultures. Three observations suggested a survival-promoting action for IL-1 alpha in nine-day-old cultures: (1) neutralizing antiserum to murine IL-1 alpha decreased neuronal survival; (2) treatment with IL-1 alpha in electrically blocked cultures increased neuronal survival; and (3) antiserum to the type I IL-1 receptor decreased neuronal survival. Treatment with VIP prevented neuronal cell death associated with the antiserum to IL-1 alpha. In contrast, treatment of one-month-old cultures with IL-1 alpha produced neuronal cell death and neutralizing antiserum to the IL-1 receptor had no effect on neuronal survival in these cultures. These experiments suggested that an IL-1-like substance was necessary for neuronal survival during a specific stage in development and that a relationship between VIP and IL-1 alpha might account in part for the neurotrophic properties of VIP. To test if VIP might be a secretagogue for IL-1, a neuron-free model system was utilized: astroglial cultures derived from cerebral cortex. VIP treatment produced a concentration-dependent (EC50: 50 pM) increase in the amount of IL-1 alpha in the medium and a decrease in cellular IL-1 alpha. Interleukin-1 beta (IL-1 beta) was also increased (EC 50: 1 nM) in the medium by VIP but without depleting IL-1 beta in the cytosol. Semi-quantitative measurements of the IL-1 alpha mRNA after VIP treatment indicated a significant but transient decrease. These data indicate that VIP produced an increase in the secretion of IL-1 alpha while depleting IL-1 alpha mRNA.

Cytokines in neurodegeneration and repair

Cytokines have diverse actions in the brain, some of which may facilitate either neurodegeneration or neuroprotection. The expression of cytokines, particularly interleukins-1 and -6 (IL-1, IL-6) and tumor necrosis factor alpha, is rapidly and markedly induced in response to experimentally induced or clinical neurodegeneration. We have demonstrated that central administration of the IL-1 receptor antagonist (IL-1ra) markedly inhibits neurodegeneration induced by focal cerebral ischaemia, local infusion of glutamate receptor agonists or traumatic brain injury in the rat. In contrast, IL-1ra offers no protection against degeneration of primary cortical neurones in culture caused by exposure to agonists of ionotrophic or metabotrophic receptors. In vivo, administration of IL-1 beta exacerbates ischaemic brain damage, whereas in cell culture, exogenous IL-1 is neuroprotective at concentrations in the nM range, an effect which appears to be mediated by release of endogenous nerve growth factor. Higher concentrations of IL-1 (microM range) are neurotoxic to neurones in culture and may mimic the involvement of IL-1 in neurodegeneration in vivo. Thus, excessive production of cytokines such as IL-1 appears to mediate experimentally induced neurodegeneration in vivo, while neuroprotective effects of low concentrations of the cytokine suggest a dual role for IL-1 in neuronal survival.