Cytokine action in the nervous system at pathophysiological versus pharmacological concentrations

Central infusion of interleukin-1 receptor antagonist blocks the reduction in social behavior produced by prior stressor exposure

Pro-inflammatory cytokines such as interleukin-1beta (IL-1beta) in the brain modulate sickness behavior in rodents, in which animals show complex changes in behavior such as the reduction of general activity, reduced social motivation, and fever response. The present studies examined the impact of lipopolysacharide (LPS) and stressor (footshock) exposure on the later expression of social behavior in Sprague-Dawley rats using two separate behavioral paradigms. In Experiment 1, a traditional test for social interaction in which animals were allowed to investigate a juvenile rat in their home cages was conducted at 4 different time points following LPS or footshock treatment. In Experiment 2, social investigation task which allowed the animals to sniff the hole connected to the other chamber where a stimulus animal was placed, but prevented physical contact, was used to measure social investigation at several time points following LPS or footshock treatment. Both systemic infusion of LPS (100 microg/kg) and 2 h footshock exposure (80 shocks, 1 mA, 5 s duration) elicited a time-dependent reduction of social interaction (Experiment 1) and investigation (Experiment 2); LPS-treated rats displayed a more profound reduction of social investigation from 2 h to 6 h after treatment, while rats exposed to footshock showed a reduction 6 h after the footshock exposure. In Experiment 3, the footshock-induced reduction of social investigation was blocked by pretreatment with IL-1 receptor antagonist (IL-1Ra; 100 microg icv) infusion. Together, these findings support a growing body of literature showing that stress-dependent changes in brain cytokines play a key role in mediating behavioral consequences of stressor exposure.

Hypothalamic integration of immune function and metabolism

The immune and neuroendocrine systems are closely involved in the regulation of metabolism at peripheral and central hypothalamic levels. In both physiological (meals) and pathological (infections, traumas and tumors) conditions immune cells are activated responding with the release of cytokines and other immune mediators (afferent signals). In the hypothalamus (central integration), cytokines influence metabolism by acting on nucleus involved in feeding and homeostasis regulation leading to the acute phase response (efferent signals) aimed to maintain the body integrity. Peripheral administration of cytokines, inoculation of tumor and induction of infection alter, by means of cytokine action, the normal pattern of food intake affecting meal size and meal number suggesting that cytokines acted differentially on specific hypothalamic neurons. The effect of cytokines-related cancer anorexia is also exerted peripherally. Increase plasma concentrations of insulin and free tryptophan and decrease gastric emptying and d-xylose absorption. In addition, in obesity an increase in interleukin (IL)-1 and IL-6 occurs in mesenteric fat tissue, which together with an increase in corticosterone, is associated with hyperglycemia, dyslipidemias and insulin resistance of obesity-related metabolic syndrome. These changes in circulating nutrients and hormones are sensed by hypothalamic neurons that influence food intake and metabolism. In anorectic tumor-bearing rats, we detected upregulation of IL-1beta and IL-1 receptor mRNA levels in the hypothalamus, a negative correlation between IL-1 concentration in cerebro-spinal fluid and food intake and high levels of hypothalamic serotonin, and these differences disappeared after tumor removal. Moreover, there is an interaction between serotonin and IL-1 in the development of cancer anorexia as well as an increase in hypothalamic dopamine and serotonin production. Immunohistochemical studies have shown a decrease in neuropeptide Y (NPY) and dopamine (DA) and an increase in serotonin concentration in tumor-bearing rats, in first- and second-order hypothalamic nuclei, while tumor resection reverted these changes and normalized food intake, suggesting negative regulation of NPY and DA systems by cytokines during anorexia, probably mediated by serotonin that appears to play a pivotal role in the regulation of food intake in cancer. Among the different forms of therapy, nutritional manipulation of diet in tumor-bearing state has been investigated. Supplementation of tumor bearing rats with omega-3 fatty acid vs. control diet delayed the appearance of tumor, reduced tumor-growth rate and volume, negated onset of anorexia, increased body weight, decreased cytokines production and increased expression of NPY and decreased alpha-melanocyte-stimulating hormone (alpha-MSH) in hypothalamic nuclei. These data suggest that omega-3 fatty acid suppressed pro-inflammatory cytokines production and improved food intake by normalizing hypothalamic food intake-related peptides and point to the possibility of a therapeutic use of these fatty acids. The sum of these data support the concept that immune cell-derived cytokines are closely related with the regulation of metabolism and have both central and peripheral actions, inducing anorexia via hypothalamic anorectic factors, including serotonin and dopamine, and inhibiting NPY leading to a reduction in food intake and body weight, emphasizing the interconnection of the immune and neuroendocrine systems in regulating metabolism during infectious process, cachexia and obesity.

Brain cytokines and disease

Cytokines (e.g. various interleukins and subfamily members, tumor necrosis factors, interferons, chemokines and growth factors) act in the brain as immunoregulators and neuromodulators. Over a decade ago, the integrative article 'Immunoregulators in the Nervous System' (Neurosci Biobehav Rev 1991; 15: 185-215) provided a comprehensive framework of pivotal issues on cytokines and the nervous system that recently have been extensively studied. Cytokine profiles in the brain, including cytokine generation and action, have been studied in multiple models associated with neuropathophysiological conditions. These include: (1) acute conditions and disorders such as stroke (cerebral ischemia or infarction and intracranial hemorrhage), traumatic brain injury, spinal cord injury and acute neuropathies; (2) chronic neurodegenerative disorders and chronic conditions, including Alzheimer's disease, Parkinson's disease, neuropathic pain, epilepsy and chronic neuropathies; (3) brain infections, including bacterial meningitis and encephalitis; (4) brain tumors; (5) neuroimmunological disorders per se, such as multiple sclerosis; (5) psychiatric disorders, including schizophrenia and depression; (6) neurological and neuropsychiatric manifestations associated with non- central nervous system (CNS) disorders such as peripheral cancer, liver, kidney and metabolic compromise, and peripheral infectious and inflammatory conditions; and (7) cytokine immunotherapy, which can be accompanied by neuropsychiatric manifestations when administered either via peripheral or brain routes. Cytokine profiles have also been studied in multiple animal models challenged with inflammatory, infectious, chemical, malignant and stressor insults. Essentially data show that cytokines play a pivotal role in multiple neuropathophysiological processes associated with different types of disorders and insults. Cytokine expression and action in the brain shows a different profile across conditions, but some similarities exist. Under a defined temporal sequence, cytokine involvement in neuroprotection or the induction of a deleterious pathophysiological cascade and in resolution/healing is proposed depending on the type of cytokine. In the brain, functional interactions among cytokines, balance between pro-inflammatory and anti-inflammatory cytokines and functional interactions with neurotransmitters and neuropeptides play a pivotal role in the overall cytokine profile, pattern of neuropathophysiological cascades, and quality and magnitude of neuropsychiatric manifestations. In this brief review various selected cytokine-related issues with relevance to the brain are discussed.

The role of TNF-alpha in amygdala kindled rats

In the present study, the interaction between epileptogenesis and the immune system were studied in a kindling model. First, the effects of a single administration of TNF-alpha (5.0 microg/kg, i.p.) on seizure and EEG activity were investigated in amygdala-kindled rats. TNF-alpha treated rats showed more prolonged epileptiformic discharges than control rats. TNF-alpha also induced a decrease in the power of delta band and an increase in theta and alpha activity. In addition, a marked increase in the power of beta and gamma band was observed. The EEG changes were most numerous in the frontal cortex and amygdala. All effects were registered 24 h after TNF-alpha administration. Finally, electrical stimulation enhanced the level of TNF-alpha in blood serum from 1.9 +/- 1.5 to 12.7 +/- 3.8 pg/ml and in brain tissue 56.8 +/- 6.0 to 109.2 +/- 6.0 pg/mg, as was determined via the ELISA method. It can be concluded that there is a mutual facilitative interaction of both epileptogenic and cytokine-derived mechanisms on this type of seizure. The changes in the power spectrum of the EEG after TNF-alpha might contribute to intensify thalamic-derived facilitation of epileptic discharge in cortical structures.

Kindling modulates the IL-1beta system, TNF-alpha, TGF-beta1, and neuropeptide mRNAs in specific brain regions

Cytokines and neuropeptides may be involved in seizure-associated processes. Following amygdala kindling in rats, we determined alterations of IL-1beta, IL-1 receptor antagonist (IL-1Ra), IL-1 receptor type I (IL-1RI), IL-1 receptor accessory proteins (IL-1R AcPs) I and II, TNF-alpha, TGF-beta1, neuropeptide Y (NPY), glycoprotein 130 (gp 130) and pro-opiomelanocortin (POMC) mRNA levels in the parietal, prefrontal and piriform cortices, amygdala, hippocampus and hypothalamus. Messenger RNAs expression in all brain regions was determined 2 h or 3 weeks following the last generalized convulsive seizure triggered from the ipsilateral kindled amygdala. The same brain region sample was used to assay for changes of all mRNA components. The results show that the 2 h-kindled group exhibited a significant up-regulation of IL-1beta, IL-1RI, TNF-alpha and TGF-beta1 mRNAs in all three cortical brain regions, amygdala and hippocampus. The largest up-regulation occurred in the prefrontal cortex (about 30-fold induction for IL-1beta and TNF-alpha mRNAs). IL-1R AcP I and II mRNA levels were also up-regulated in the cortical regions. No changes in IL-1beta, IL-1RI or TNF-alpha mRNA levels occurred in the 3 week-kindled group. NPY mRNA levels increased in the hippocampus, prefrontal and piriform cortices in the 2 h-kindled group, while IL-1Ra, gp 130, or POMC mRNA levels did not change in any group. The overall profile of mRNA changes shows specificity of transcriptional modulation induced by amygdala kindling. The data support a role of cytokines and NPY in the adaptive mechanisms associated with generalized seizure activity, with implications for neuroprotection, neuronal dysfunction and vulnerability associated with epileptic activity.

1998 Curt P. Richter Award. Brain mechanisms in cytokine-induced anorexia

Our research focuses on the mechanisms underlying cytokine action in the central nervous system (CNS) using an integrative and multidisciplinary strategy organized through supracellular (behavioral analysis by computerized monitoring systems), cellular (extracellular and intracellular neurophysiological recording), and molecular (patch-clamp recording, and DNA, RNA and protein analyses) approaches. An integrative strategy that combines computerized meal pattern analyses with cellular and molecular biology approaches allows the study of underlying brain mechanisms in cytokine- and disease-associated anorexia. This paper presents a comprehensive discussion of our laboratory's previously published data on brain mechanisms involved in cytokine-induced anorexia including the relevance of meal pattern analysis (meal size, meal duration, meal frequency, intermeal intervals), modulation of hypothalamic neuronal activity, molecular processes involving ionic conductances, cytokine-cytokine and cytokine-peptide interactions, and modulation of cytokine and peptide/neuropeptide system components (ligands, endogenous inhibitors, receptor subtypes, signal transduction molecules, intracellular mediators) and cytokine feedback systems.

Cytokine-induced anorexia. Behavioral, cellular, and molecular mechanisms

Cytokines induce anorexia. Recent issues concerning mechanistic aspects are: (1) Cytokines induce anorexia through different modes of behavioral action, that is, by affecting meal size, meal duration, and meal frequency differentially. Profiles also depend on the concentration or dosage. (2) The interface between the periphery and brain. Specific cytokines may be transported from the periphery to the brain. Cytokines generate mediators that can act on peripheral and/or brain target sites. Cerebrovasculature endothelium can also generate signals to modulate neural activities. Evidence indicates that the proposed vagal afferent signaling requires reassessment. Because of paracrine and autocrine actions, local cytokine production within the brain can induce anorexia. (3) Cytokines act directly on hypothalamic neurons proposed to participate in feeding. (4) Cytokine<-->cytokine and cytokine<-->peptide/neurotransmitter interactions are critical; for example, cytokines interact to induce anorexia synergistically, neuropeptide Y<-->cytokine interactions are antagonist, and cytokine<-->neurotransmitter and cytokine<-->leptin<-->neuropeptide Y<-->CRH-glucocorticoid and other endocrine interactions are important. A leptin receptor is related to gp 130, a signal transducer among interleukin (IL)-6 subfamily receptors; gp 130 and related molecules may be an interface for feeding control in health and disease. Various cytokines upregulate leptin and gp 130. An integrative approach combining computerized meal pattern analyses with cellular and molecular approaches is being used to characterize mechanisms (ligands, receptors, transducing molecules, and intracellular mediators) involved in cytokine-induced anorexia.