Type 1 interleukin-1 receptor in the rat brain: distribution, regulation, and relationship to sites of IL-1-induced cellular activation

Evidence that the bed nucleus of the stria terminalis contributes to the modulation of hypophysiotropic corticotropin-releasing factor cell responses to systemic interleukin-1beta

Systemic infection activates the hypothalamic-pituitary-adrenal (HPA) axis, and brainstem catecholamine cells have been shown to contribute to this response. However, recent work also suggests an important role for the central amygdala (CeA). Because direct connections between the CeA and the hypothalamic apex of the HPA axis are minimal, the present study investigated whether the bed nucleus of the stria terminalis (BNST) might act as a relay between them. This was done by using an animal model of acute systemic infection involving intravascular delivery of the proinflammatory cytokine interleukin-1beta (IL-1beta, 1 microg/kg). Unilateral ibotenic acid lesions encompassing the ventral BNST significantly reduced both IL-1beta-induced increases in Fos immunoreactivity in corticotropin-releasing factor (CRF) cells of the hypothalamic paraventricular nucleus (PVN) and corresponding increases in adrenocorticotropic hormone (ACTH) secretion. Similar lesions had no effect on CRF cell responses to physical restraint, suggesting that the effects of BNST lesions were not due to a nonspecific effect on stress responses. In further studies, we examined the functional connections between PVN, BNST, and CeA by combining retrograde tracing with mapping of IL-1beta-induced increases in Fos in BNST and CeA cells. In the case of the BNST, these studies showed that systemic IL-1beta administration recruits ventral BNST cells that project directly to the PVN. In the case of the CeA, the results obtained were consistent with an arrangement whereby lateral CeA cells recruited by systemic IL-1beta could regulate the activity of medial CeA cells projecting directly to the BNST. In conclusion, the present findings are consistent with the hypothesis that the BNST acts as a relay between the CeA and PVN, thereby contributing to CeA modulation of hypophysiotropic CRF cell responses to systemic administration of IL-1beta.

Circulating cytokines and endotoxin are not necessary for the activation of the sickness or corticosterone response produced by peripheral E. coli challenge

Peripheral administration of a variety of inflammatory stimuli, such as endotoxin or cytokines, induces an orchestrated set of brain-mediated events referred to as the sickness response. The mechanism for how immune products signal the brain is not clear, but accumulating evidence supports the existence of neural as well as blood-borne pathways. Although endotoxin or cytokine administration results in sickness responses, few data exist regarding the role of circulating endotoxin or cytokines in the induction of sickness during a real bacterial infection. Thus the present studies examined whether subcutaneously administered Escherichia coli can activate sickness responses and whether circulating endotoxin and/or proinflammatory cytokines are a prerequisite for these responses. Male Sprague-Dawley rats were injected subcutaneously with one of three doses (2.5 x 10(7), 2.5 x 10(8), 2.5 x 10(9) colony-forming units) of replicating E. coli, a ubiquitous bacterial strain, or vehicle. Core body temperature (Tc) and activity were measured for 3 days after the injection. A second set of groups of animals were killed 3, 6, 12, 18, 24, and 48 h after the injection, and blood samples and brains were collected. Injections dose dependently and consistently increased Tc and decreased activity, with increases in Tc beginning 4 h after the injection. In addition, E. coli significantly increased serum interleukin (IL)-1beta, IL-6, and tumor necrosis factor-alpha and brain IL-1beta levels beginning at the 6-h time point. Corticosterone and endotoxin were first elevated in the circulation at 3 and 18 h after the injection, respectively. Because fever onset preceded brain cytokine induction, we also examined cytokine levels in the serum, brain, and inflammation site 2 and 4 h after injection. Cytokines were elevated at the inflammation site but were not detectable in the serum or brain at 2 and 4 h. We conclude that subcutaneous injection of replicating E. coli induces a consistent and naturalistic infection that includes features of the sickness response as well as increases in circulating, brain, and inflammation site tissue cytokines. In addition, injection of replicating E. coli produces a robust fever and corticosterone response at a time when there are no detectable increases in circulating cytokines or endotoxin. These results suggest that elevated levels of circulating cytokines and endotoxin are not necessary for the activation of the sickness or corticosterone response. Therefore, fever, activity reduction, and corticosterone elevation induced by E. coli infection may have been evoked by a neural, rather than a humoral, pathway from the periphery to the brain.

Distribution of interleukin-1 receptor in chicken and quail brain

Interleukin 1 isoforms (IL-1) are major regulators of vertebrate immune responses. In the mammalian CNS, this function is reflected in physiological and anatomical evidence implicating IL-1 in a suite of behaviors associated with sickness. Although birds show sickness behavior, a parallel role of IL-1 in birds has not been investigated. As proinflammatory effects of IL-1 are mediated via the IL-1 type I receptor (IL-1RI), we investigated the distribution of IL-1RI protein and mRNA after lipopolysaccharide challenge in brains of two avian species, the chicken and Japanese quail. In some respects, the neuroanatomic distribution of IL-1R mRNA and protein in chicken and Japanese quail resembled that reported in mammals and was consistent with its putative role in the physiology and behavior of sickness. For example, we found IL-1RI mRNA or IL-1RI immunoreactivity in lemnothalamic visual projection areas of the pallium, surrounding blood vessels in pallial areas, in the dorsomedial nucleus of the hypothalamus, in the nucleus taenia, in cerebeller Purkinje cells and the motor components of the trigeminal and vagus nuclei. However, in contrast to mammals, we did not find evidence of IL1-RI receptors in medial or lateral pallial structures, paraventricular nucleus, areas homologous to the arcuate nucleus, the choroid plexus, organum vasculosum of the lamina terminalis or the reticular activating system. The distribution of IL-1RI suggests that a role for IL-1 in sickness behavior is conserved in birds, but that roles in other putative mammalian functions (e.g. hypothalamic-pituitary-adrenal and gonadal axes regulation, transport through barrier-related tissues, arousal) may differ.

IL-1 plays an important role in lipid metabolism by regulating insulin levels under physiological conditions

IL-1 is a proinflammatory cytokine that plays important roles in inflammation. However, the role of this cytokine under physiological conditions is not known completely. In this paper, we analyzed the role of IL-1 in maintaining body weight because IL-1 receptor antagonist-deficient (IL-1Ra-/-) mice, in which excess IL-1 signaling may be induced, show a lean phenotype. Body fat accumulation was impaired in IL-1Ra-/- mice, but feeding behavior, expression of hypothalamic factors involved in feeding control, energy expenditure, and heat production were normal. When IL-1Ra-/- mice were treated with monosodium glutamate (MSG), which causes obesity in wild-type mice by ablating cells in the hypothalamic arcuate nucleus, they were resistant to obesity, indicating that excess IL-1 signaling antagonizes the effect of MSG-sensitive neuron deficiency. IL-1Ra-/- mice showed decreased weight gain when they were fed the same amount of food as wild-type mice, and lipid accumulation remained impaired even when they were fed a high-fat diet. Interestingly, serum insulin levels and lipase activity were low in IL-1Ra-/- mice, and the insulin levels were low in contrast to wild-type mice after MSG treatment. These observations suggest that IL-1 plays an important role in lipid metabolism by regulating insulin levels and lipase activity under physiological conditions.

Transgenic mice expressing green fluorescent protein under the control of the melanocortin-4 receptor promoter

The melanocortin-4 receptor (MC4-R) is an important regulator of energy homeostasis, and evidence suggests that MC4-R-expressing neurons are downstream targets of leptin action. MC4-Rs are broadly expressed in the CNS, and the distribution of MC4-R mRNA has been analyzed most extensively in the rat. However, relatively little is known concerning chemical profiles of MC4-R-expressing neurons. The extent to which central melanocortins act presynaptically or postsynaptically on MC4-Rs is also unknown. To address these issues, we have generated a transgenic mouse line expressing green fluorescent protein (GFP) under the control of the MC4-R promoter, using a modified bacterial artificial chromosome. We have confirmed that the CNS distribution of GFP-producing cells is identical to that of MC4-R mRNA in wild-type mice and that nearly all GFP-producing cells coexpress MC4-R mRNA. For example, cells coexpressing GFP and MC4-R mRNA were distributed in the paraventricular hypothalamic nucleus (PVH) and the dorsal motor nucleus of the vagus (DMV). MC4-R promotor-driven GFP expression was found in PVH cells producing thyrotropin-releasing hormone and in cholinergic DMV cells. Finally, we have observed that a synthetic MC3/4-R agonist, MT-II, depolarizes some GFP-expressing cells, suggesting that MC4-Rs function postsynaptically in some instances and may function presynaptically in others. These studies extend our knowledge of the distribution and function of the MC4-R. The transgenic mouse line should be useful for future studies on the role of melanocortin signaling in regulating feeding behavior and autonomic homeostasis.

IL-1 beta depresses respiration and anoxic survival via a prostaglandin-dependent pathway in neonatal rats

IL-1 beta has been proposed to be an important mediator linking infection, apnea, and sudden infant death syndrome. We hypothesized that IL-1 beta acts in this capacity by depressing brainstem respiratory neurons via a prostaglandin-dependent pathway. For studying the effects of IL-1 beta on respiration as well as the mechanism underlying its actions, 7-d-old rats received an initial injection (i.p.) of NaCl or a cyclooxygenase inhibitor (indomethacin, 10 mg/kg) followed by a second injection (i.p.) at 30 min of NaCl, recombinant rat IL-1 beta (10 microg/kg), or lipopolysaccharide (LPS; 100 microg/kg). Respiration during normoxia and in response to anoxia (100% N2) was examined at 60 min after the second injection using flow and barometric plethysmography. Animals given IL-1 beta breathed more slowly and died more often after anoxia. LPS also reduced the rats' ability to autoresuscitate and survive an anoxic challenge. Indomethacin prevented the depressive effects during normoxia and the adverse effects on survival. For investigating drug-induced changes in central respiratory activity, IL- 1 beta (1.0 or 1.25 ng/mL) and prostaglandin E2 (5 or 20 microg/L) was applied to the brainstem-spinal cord preparation of 0- to 4-d-old rats. Whereas IL-1 beta exerted no effect on respiration measured at the C4 ventral root during a 60-min period, prostaglandin E2 reversibly inhibited respiratory activity. These findings suggest that IL-1 beta does not inhibit respiratory neurons directly but may depress breathing and hypoxic defense via a prostaglandin-mediated mechanism.

Descending pathways from the paraventricular nucleus contribute to the recruitment of brainstem nuclei following a systemic immune challenge

Hypothalamic nuclei, particularly the paraventricular nuclei (PVN), are important brain sites responsible for central nervous system responses during an immune challenge. The brainstem catecholamine cells of the nucleus tractus solitarius (NTS) and ventrolateral medulla (VLM) have been shown to play critical roles in relaying systemic immune signals to the PVN. However, whilst it is well recognised that PVN divisions also innervate the NTS and VLM, it is not known whether descending PVN pathways can modulate the recruitment of brainstem cells during an immune challenge. Using systemic administration of the proinflammatory cytokine interleukin-1beta, in combination with Fos immunolabelling, we firstly investigated the effect of PVN lesions on NTS and VLM catecholamine and non-catecholamine cell responses. We found that ibotenic acid lesions of the PVN significantly reduced numbers of Fos-positive non-catecholamine, noradrenergic and adrenergic cells observable in the VLM and NTS after interleukin-1beta administration. We then investigated the origins of descending inputs to the VLM and NTS, activated by systemic interleukin-1beta, by mapping the distribution of Fos-positive retrogradely-labelled cells in divisions of the PVN after iontophoretically depositing choleratoxin-b subunit into the NTS or VLM one week prior to interleukin-1beta administration. We found that, after either NTS or VLM deposits, the majority of retrogradely-labelled Fos-positive cells activated by interleukin-1beta were localised in the medial and lateral parvocellular PVN divisions. Retrogradely-labelled Fos-positive cells were also observed in the NTS after VLM deposits, and in the VLM after NTS tracer deposits, suggesting reciprocal communication between these two nuclei after systemic interleukin-1beta. Thus the present study shows that the PVN has the capacity to modulate NTS and VLM responses after an immune challenge and that these may result from descending projections arising in the medial and lateral PVN divisions. These findings suggest that central nervous system responses to an immune challenge are likely to involve complex reciprocal connections between the PVN and the brainstem as well as between brainstem nuclei themselves.

A decrease in interleukin-1 receptor antagonist expression in the prefrontal cortex of schizophrenic patients

Interleukin-1 (IL-1) mediates psychological stress responses by regulating monoamine metabolism and secretion of corticotropin-releasing factor, and is therefore, implicated in various psychiatric diseases. To evaluate the contribution of IL-1 signaling to the brain pathology of schizophrenia, we measured protein and/or mRNA levels for IL-1beta and endogenous IL-1 receptor antagonist (IL-1RA) in the postmortem brain tissues of prefrontal and parietal cortex, putamen, and hypothalamus. Both protein and mRNA levels of IL-1RA were specifically decreased in the prefrontal cortex of schizophrenic patients, whereas IL-1beta levels were not significantly altered in all the regions examined. The IL-1RA decrease was not correlated with the dose of antipsychotics given to patients. There was no influence of this illness on protein levels for IL-1 receptor type 1 in the prefrontal cortex, either. In contrast, IL-1RA serum levels were increased in schizophrenic patients, especially in drug-free patients, as reported previously. These findings suggest that chronic schizophrenia down-regulates IL-1RA production the prefrontal cortex, irrespective of its impact on the periphery. IL-1RA reduction might reflect an immunopathologic trait of the prefrontal region in schizophrenic patients.

Central interleukin-1beta antibody increases renal and splenic sympathetic nerve discharge

We tested the hypothesis that intracerebroventricular (lateral ventricle) administration of interleukin-1beta (IL-1beta) antibody increases the level of sympathetic nerve discharge (SND) in alpha-chloralose-anesthetized rats. Mean arterial pressure (MAP), heart rate (HR), and SND (splenic and renal) were recorded before (Preinfusion), during (25 min), and for 45 min after infusion of IL-1beta antibody (15 microg, 50 microl icv) in baroreceptor-intact (intact) and sinoaortic-denervated (SAD) rats. The following observations were made. First, intracerebroventricular infusion of IL-1beta antibody (but not saline and IgG) significantly increased MAP and the pressor response was higher in SAD compared with intact rats. Second, renal and splenic SND were significantly increased during and after intracerebroventricular IL-1beta antibody infusion and sympathoexcitatory responses were higher in SAD compared with intact rats. Third, intracerebroventricular administration of a single dose of IL-1beta antibody (15 microg, 5 microl for 2 min) significantly increased splenic and renal SND in intact rats. These results suggest that under the conditions of the present experiments central neural IL-1beta plays a role in the tonic regulation of SND and arterial blood pressure.

Expression of melanocortin 4 receptor mRNA in the central nervous system of the rat

The melanocortin 4 receptor (MC4-R) plays a pivotal role in maintaining energy homeostasis in rodents and humans. For example, MC4-R deletion or mutation results in obesity, hyperphagia, and insulin resistance. Additionally, subsets of leptin-induced autonomic responses can be blocked by melanocortin receptor antagonism, suggesting that MC4-R-expressing neurons are downstream targets of leptin. However, the critical autonomic control sites expressing MC4-Rs are still unclear. In the present study, we systematically examined the distribution of MC4-R mRNA in the adult rat central nervous system, including the spinal cord, by using in situ hybridization histochemistry (ISHH) with a novel cRNA probe. Autonomic control sites expressing MC4-R mRNA in the hypothalamus included the anteroventral periventricular, ventromedial preoptic, median preoptic, paraventricular, dorsomedial, and arcuate nuclei. The subfornical organ, dorsal hypothalamic, perifornical, and posterior hypothalamic areas were also observed to express MC4-R mRNA. Within extrahypothalamic autonomic control sites, MC4-R-specific hybridization was evident in the infralimbic and insular cortices, bed nucleus of the stria terminalis, central nucleus of the amygdala, periaqueductal gray, lateral parabrachial nucleus, nucleus of the solitary tract, dorsal motor nucleus of the vagus (DMV), and intermediolateral nucleus of the spinal cord (IML). By using dual-label ISHH, we confirmed that the cells expressing MC4-R mRNA in the IML and DMV were autonomic preganglionic neurons as cells in both sites coexpressed choline acetyltransferase mRNA. The distribution of MC4-R mRNA is consistent with the proposed roles of central melanocortin systems in feeding and autonomic regulation.

Cytokines in depression and heart failure

OBJECTIVE

There is a convincing body of evidence linking depression, cardiovascular disease, and mortality. There is also growing evidence that depression is a risk factor for congestive heart failure (CHF) and that CHF patients with major depression have higher rates of mortality and repeat hospitalizations. Currently there are no proposed neurobiological or neuroimmune mechanisms for the comorbidity of heart failure and depression.

METHODS

This review focuses on the recent literature about the role of cytokines in CHF and depression as separate conditions. This review also attempts to identify the overlapping immunological mechanisms that have a potential for future research in the pathophysiology of comorbid depression and CHF.

RESULTS

Results of current studies suggest that cytokines exert deleterious effects on the heart and that soluble tumor necrosis factor (TNF) receptor 2 leads to reversal of the cardiotoxic effects of TNF, although the clinical significance of this is unclear. Major depression has been associated with alteration of various aspects of the innate immune system, including cellular components (such as microphages, neutrophils, and natural killer cells) and soluble mediators (such as acute-phase reaction proteins and cytokines). It is inconclusive whether antidepressants have immunoregulatory effects.

CONCLUSIONS

The literature has not yet addressed the role of cytokines in comorbid depression and CHF. But cytokines may provide a new avenue in understanding brain-body interaction in depression and heart failure.

Endothelial activation is an intermediate step for peripheral lipopolysaccharide induced activation of paraventricular nucleus

Peripheral injection of bacterial endotoxin lipopolysaccharide (LPS) activates the paraventricular nuclei of the hypothalamus (PVN), and consequently the hypothalamus-pituitary adrenal axis. Inflammatory cytokine interleukin-1 (IL-1) has been considered as a key mediator that translates the peripheral LPS stimulation into neuronal activation in the PVN. Several studies attempting to localize the expression of receptors for IL-1 (IL-1R), however, have failed to detect IL-1R on PVN neurons. It remains unclear, therefore, how IL-1 might stimulate the neurons of the PVN. In this study, we traced the cellular responsiveness to IL-1 by measuring the mRNA production of the cytokine responsive gene IkappaBalpha in the PVN. After either peripheral injection LPS or intracerebroventricular (i.c.v.) injection of IL-1beta, IkappaBalpha mRNA was found mostly in endothelial cells of the brain with the highest level of expression in PVN blood vessels. In addition, both injections also induced the expression of cyclooxygenase-2 in brain endothelial cells. Pretreatment with indomethacin, a cyclooxygenase inhibitor, blocked LPS and IL-1 induced neuronal activation in the PVN, but did not reduce the induction of IkappaBalpha in PVN endothelium. These results show that IL-1 acting on the endothelial cells of the brain, particularly in the PVN, may be an intermediate step relating peripheral immune signals to the brain.

Differential induction of interleukin-1beta and tumour necrosis factor-alpha may account for specific patterns of leukocyte recruitment in the brain

In peripheral tissue, IL-1beta has been shown to induce TNFalpha expression and vice versa, resulting in mixed neutrophil and mononuclear cell recruitment to the site of injury. This has led to the concept of crosstalk in peripheral cytokine signalling pathways. In the brain parenchyma, however, restricted patterns of leukocyte recruitment following the focal injection of pro-inflammatory agents into the brain are observed. This study investigates the expression of the principal pro-inflammatory cytokines--IL-1beta and TNFalpha--in the brain after IL-1beta, TNFalpha, NMDA or endotoxin injection into the brain parenchyma of rats. Each of these agents gives rise to a distinct pattern of acute leukocyte recruitment at 24 h. We found that IL-1beta induces de novo synthesis of additional IL-1beta but not TNFalpha, as determined by RT-PCR and ELISA, and TNFalpha does not induce either itself or IL-1beta. Injection of NMDA results in IL-1beta, but not TNFalpha up-regulation. Injection of IL-1beta or NMDA is associated with neutrophil recruitment whereas injection of TNFalpha is associated with mononuclear cell recruitment. Following injection of endotoxin, both TNFalpha and IL-1beta levels are elevated and neutrophils and mononuclear cells are recruited to the brain. These data suggest that the signalling pathways that are present in the periphery are modified in the brain and that differential induction of TNFalpha and IL-1beta may have a role in the atypical pattern of leukocyte recruitment observed in the brain.

Systemic administration of interleukin-1beta activates select populations of central amygdala afferents

The central nucleus of the amygdala (CeA) is activated robustly by an immune challenge such as the systemic administration of the proinflammatory cytokine interleukin-1beta (IL-1beta). Because IL-1beta is not believed to cross the blood-brain barrier in any significant amount, it is likely that IL-1beta elicits CeA cell recruitment by means of activation of afferents to the CeA. However, although many studies have investigated the origins of afferent inputs to the CeA, we do not know which of these also respond to IL-1beta. Therefore, to identify candidate neurons responsible for the recruitment of CeA cells by an immune challenge, we iontophoretically deposited a retrograde tracer, cholera toxin b-subunit (CTb), into the CeA of rats 7 days before systemic delivery of IL-1beta (1 microg/kg, i.a.). By using combined immunohistochemistry, we then quantified the number of Fos-positive CTb cells in six major regions known to innervate the CeA. These included the medial prefrontal cortex, paraventricular thalamus (PVT), ventral tegmental area, parabrachial nucleus (PB), nucleus tractus solitarius, and ventrolateral medulla. Our results show that after deposit of CTb into the CeA, the majority of double-labeled cells were located in the PB and the PVT, suggesting that CeA cell activation by systemic IL-1beta is likely to arise predominantly from cell bodies located in these regions. These findings may have significant implications in determining the central pathways involved in generating acute central responses to a systemic immune challenge.

Intracerebroventricular but not intravenous interleukin-1beta induces widespread vascular-mediated leukocyte infiltration and immune signal mRNA expression followed by brain-wide glial activation

Interleukin-1beta (IL-1beta) is a pro-inflammatory cytokine that appears in brain and cerebrospinal fluid following peripheral immune challenges and central infections or injury. We examined the consequences of i.c.v. infusion of IL-1beta on mRNA expression of several immune markers and on recruitment of peripheral leukocytes. Awake rats were infused with IL-1beta (100 ng/rat) into the lateral ventricle, and 0.5, 2, 4, 8, 12, or 24 h later, animals were killed and their fresh-frozen brains processed for in situ hybridization and immunohistochemistry. Widespread vascular expression of inhibitory factor kappa(B)alpha (Ikappa(B)alpha, marker of nuclear factor kappa(B)alpha transcriptional activity) and inducible cyclooxygenase (COX-2) mRNAs at 0.5-2 h was credited to movement of IL-1beta along ventricular, subarachnoid, and perivascular pathways to target endothelia that express type 1 IL-1 receptor mRNA. Induction of monocyte chemoattractant protein-1 mRNA and intercellular adhesion molecule-1 (ICAM-1) immunostaining on endothelia began at 0.5-2 h. Leukocytes (neutrophils and monocytes, recognized by morphology and CD45 and ED1 immunostaining) appeared in meninges and blood vessels at 2-4 h and diffusely penetrated the parenchyma at 8-24 h. The leukocytes strongly expressed IL-1beta and inducible nitric oxide synthase mRNAs. Beginning at 4-12 h, astrocytes (glial acidic fibrillary protein mRNA and protein and c-fos mRNA) and microglia (ionized calcium-binding adaptor molecule 1 mRNA and protein) showed widespread activation. Other rats received i.v. IL-1beta (6 microg/kg). Their brains showed induction of Ikappa(B)alpha and COX-2 mRNAs in the vasculature at 2 h but none of the other sequelae. In summary, our data indicate that IL-1beta in the cerebrospinal fluid reaches its target receptors on the endothelia via perivascular volume transmission, up-regulates ICAM-1, and triggers a targeted leukocyte emigration and widespread glial activation stimulated perhaps by pro-inflammatory molecules expressed by leukocytes. The dramatic difference between i.c.v. and i.v. routes of administration underscores the potency of IL-1beta within the brain to dynamically affect the cellular trafficking component of 'immune privilege'.

Effects of interleukin-1 beta on the steroid-induced luteinizing hormone surge: role of norepinephrine in the medial preoptic area

Interleukin-1beta (IL-1beta), a cytokine, is known to inhibit the preovulatory surge of luteinizing hormone (LH); however, the mechanism by which it does so is unclear. This study was done to see if this effect is mediated through hypothalamic catecholamines. Adult female Sprague-Dawley rats were ovariectomized and implanted with a push-pull cannula in the medial preoptic area (MPA) of the hypothalamus. They were injected subcutaneously with 30 microg of Estradiol on the day 8 after surgery and with 2mg of Progesterone on day 10 at 1000 h. On the day of perfusion (day 10), the rats were injected with IL-1beta or its vehicle at 1300 h. Perfusate samples from the MPA and blood samples from a jugular catheter were collected from 1300 to 1800 h. Catecholamine concentrations in the perfusate were measured using high performance liquid chromatography (HPLC)-EC and LH levels in the serum using RIA. Norepinephrine release in the MPA of control rats increased significantly at 1530, 1600, and 1630 h paralelling an increase in LH at 1600 h. In contrast, IL-1beta treatment blocked the LH surge and the rise in norepinephrine release in the MPA. No changes were observed in dopamine release, both in control and IL-treated animals. These results demonstrate for the first time that IL-induced suppression of the LH surge is probably mediated through inhibition of norepinephrine release in the MPA.

Sympathoexcitation to intravenous interleukin-1beta is dependent on forebrain neural circuits

We investigated the contributions of forebrain, brain stem, and spinal neural circuits to interleukin (IL)-1beta-induced sympathetic nerve discharge (SND) responses in alpha-chloralose-anesthetized rats. Lumbar and splenic SND responses were determined in spinal cord-transected (first cervical vertebra, C1), midbrain-transected (superior colliculus), and sham-transected rats before and for 60 min after intravenous IL-1beta (285 ng/kg). The observations made were the following: 1) lumbar and splenic SND were significantly increased after IL-1beta in sham C1-transected rats but were unchanged after IL-1beta in C1-transected rats; 2) intrathecal administration of DL-homocysteic acid (10 ng) increased SND in C1-transected rats; 3) lumbar and splenic SND were significantly increased after IL-1beta in sham- but not midbrain-transected rats; and 4) midbrain transection did not alter the pattern of lumbar and splenic SND, demonstrating the integrity of brain stem sympathetic neural circuits after decerebration. These results demonstrate that an intact forebrain is required for mediating lumbar and splenic sympathoexcitatory responses to intravenous IL-1beta, thereby providing new information about the organization of neural circuits responsible for mediating sympathetic-immune interactions.

Distinct brain vascular cell types manifest inducible cyclooxygenase expression as a function of the strength and nature of immune insults

Induced prostanoid synthesis by cells associated with the cerebral vasculature has been implicated in mediating immune system influences on the CNS, but the cell type(s) involved remain unsettled. To determine whether this might derive from differences in the nature and intensity of the stimuli used to model immune insults, immunochemical and hybridization histochemical methods were used to monitor cyclooxygenase-2 (COX-2) expression alone, or in conjunction with endothelial, perivascular, and glial cell markers, in brains of rats treated with varying doses of interleukin-1 (IL-1) or bacterial lipopolysaccharide (LPS). Vehicle-treated animals displayed weak COX-2 expression in the meninges, choroid plexus, and larger blood vessels. Rats challenged intravenously with IL-1beta (1.87-30 microgram/kg) showed a marked increase in the number of vascular-associated cells displaying COX-2-immunoreactivity (ir). More than 90% stained positively for the ED2 macrophage differentiation antigen, identifying them as perivascular cells, whereas none coexpressed endothelial or glial cell markers. Low doses of LPS (0.1 microgram/kg) elicited a similar response profile, but higher doses (2-100 microgram/kg) provoked COX-2 expression in a progressively greater number of cells exhibiting distinct round or multipolar morphologies, corresponding to cells expressing endothelial (RECA-1) or perivascular (ED2) cell antigens, respectively. Similarly, ultrastructural analysis localized COX-2-ir to the perinuclear region of endothelial cells of LPS-treated but not IL-1-treated rats. We conclude that perivascular cells exhibit the lower threshold to COX-2 expression in response to either IL-1 or endotoxin treatment, and that enzyme expression by endothelial cells requires one or more facets of the more complex immune stimulus presented by LPS.

Involvement of the arcuate nucleus of the hypothalamus in interleukin-1-induced anorexia

Cytokine-mediated anorexia is a component of "sickness behavior" and presents a significant obstacle in the treatment of chronic illnesses. We hypothesized an involvement of the hypothalamic arcuate nucleus (ARH) in mediating the anorexic effects of a systemic interleukin-1 (IL-1) challenge based on its content of peptidergic neurons involved in feeding, its expression of IL-1 receptors and its sensitivity to systemic IL-1. IL-1 (6 microg/kg, i.v.) was found to induce Fos expression in both pro-opiomelanocortin- and neuropeptide Y-expressing neurons in and around the ARH. Contrary to expectations, rats that had sustained lesions of the arcuate nucleus, produced by neonatal monosodium glutamate treatment, displayed a more pronounced suppression (by 25%) of food intake than nonlesioned controls when treated with IL-1 after a 20 hr fast. To confirm and further characterize this unexpected result, a second ablation method was used in a similar paradigm. Animals bearing knife cuts designed to sever major ARH projections displayed an even more accentuated loss of appetite (by 60%, relative to controls) in response to systemic IL-1. This effect exhibited at least some degree of specificity, because the knife cuts did not alter either IL-1 effects on another centrally mediated acute phase response (fever) or the anorexia produced by an alternate agent, fenfluramine. These results fail to support the hypothesized ARH mediation of IL-1-induced anorexia and may suggest rather that the net output of this cell group may serve normally to restrain cytokine-induced reductions in food intake.

Glucagon-like peptide-1 receptor stimulation increases blood pressure and heart rate and activates autonomic regulatory neurons

Glucagon-like peptide-1 (GLP-1) released from the gut functions as an incretin that stimulates insulin secretion. GLP-1 is also a brain neuropeptide that controls feeding and drinking behavior and gastric emptying and elicits neuroendocrine responses including development of conditioned taste aversion. Although GLP-1 receptor (GLP-1R) agonists are under development for the treatment of diabetes, GLP-1 administration may increase blood pressure and heart rate in vivo. We report here that centrally and peripherally administered GLP-1R agonists dose-dependently increased blood pressure and heart rate. GLP-1R activation induced c-fos expression in the adrenal medulla and neurons in autonomic control sites in the rat brain, including medullary catecholamine neurons providing input to sympathetic preganglionic neurons. Furthermore, GLP-1R agonists rapidly activated tyrosine hydroxylase transcription in brainstem catecholamine neurons. These findings suggest that the central GLP-1 system represents a regulator of sympathetic outflow leading to downstream activation of cardiovascular responses in vivo.

The age-related increase in IL-1 type I receptor in rat hippocampus is coupled with an increase in caspase-3 activation

Evidence from several studies indicates that expression of interleukin-1beta (IL-1beta) and IL-1 type I receptor is particularly high in hippocampus, and it has recently been shown that the concentration of IL-1beta is increased in the hippocampus of the aged rat. Here we report that this increase is coupled with an increase in expression of IL-1 type I receptor and increased activity of IL-1 receptor-associated kinase. The evidence presented indicates that the age-related increase in activity of the mitogen-activated protein kinases, Jun N-terminal kinase (JNK) and p38, was accompanied by enhanced caspase-3 activity. Analysis of colocalization of activated caspase-3 with activated p38 (p-p38) suggested that p-p38 was necessary for activation of caspase-3; while in vitro analysis indicated that the IL-1beta-induced increase in caspase-3 activity was abrogated by the p38 inhibitor, SB203580. The IL-1beta-induced increase in caspase-3 activity in vitro was also abrogated by vasoactive intestinal peptide, which is a JNK inhibitor; however, colocalization of activated JNK (p-JNK) and activated caspase-3 did not clearly identify JNK as an upstream activator of caspase-3. We propose that these changes are indicative of cell death in aged hippocampus and suggest that they contribute to the age-related decrease in long-term potentiation in perforant path granule cell synapses.

Site-specific modulation of LPS-induced fever and interleukin-1 beta expression in rats by interleukin-10

Bacterial lipopolysaccharide (LPS) induces fever that is mediated by pyrogenic cytokines such as interleukin (IL)-1 beta. We hypothesized that the anti-inflammatory cytokine IL-10 modulates the febrile response to LPS by suppressing the production of pyrogenic cytokines. In rats, intravenous but not intracerebroventricular infusion of IL-10 was found to attenuate fever induced by peripheral administration of LPS (10 microg/kg iv). IL-10 also suppressed LPS-induced IL-1 beta production in peripheral tissues and in the brain stem. In contrast, central administration of IL-10 attenuated the febrile response to central LPS (60 ng/rat icv) and decreased IL-1 beta production in the hypothalamus and brain stem but not in peripheral tissues and plasma. Furthermore, intravenous LPS upregulated expression of IL-10 receptor (IL-10R1) mRNA in the liver, whereas intracerebroventricular LPS enhanced IL-10R1 mRNA in the hypothalamus. We conclude that IL-10 modulates the febrile response by acting in the periphery or in the brain dependent on the primary site of inflammation and that its mechanism of action most likely involves inhibition of local IL-1 beta production.

Neural-immune gut-brain communication in the anorexia of disease

Peripheral administration of toxic bacterial products and cytokines have been used to model the immunological, physiological, and behavioral responses to infection, including the anorexia of disease. The vagus nerve is the major neuroanatomic linkage between gut sites exposed to peripheral endotoxins and cytokines and the central nervous system regions that mediate the control of food intake, and thus has been a major research focus of the neurobiological approach to understanding cytokine-induced anorexia. Molecular biological and neurophysiologic evidence demonstrates that peripheral anorectic doses of cytokines and endotoxins elicit significant increases in neural activation at multiple peripheral and central levels of the gut-brain axis and in some cases may modify the neural processing of meal-related gastrointestinal signals that contribute to the negative feedback control of ingestion. However, behavioral studies of the anorectic effects of peripheral cytokines and endotoxins have shown that neither vagal nor splanchnic visceral afferent fibers supplying the gut are necessary for the reduction of food intake in these models. These data do not rule out 1) the potential contribution of supradiaphragmatic vagal afferents or 2) a modulatory role for immune-stimulated gut vagal afferent signals in the expression of cytokine and endotoxin-induced anorexia in the intact organism.

Antidepressant reversal of interferon-alpha-induced anhedonia

Interferon-alpha(IFN-alpha) is used clinically in the treatment of several pathologies such as hepatitis C and various cancers. The positive therapeutic potential is however often limited by negative secondary effects which include major depression, one of the cardinal symptoms of which is anhedonia which has been operationalized as a decreased sensitivity to rewards (inability to experience pleasure). Previous studies have demonstrated the existence of anhedonia in rats following an acute injection of IFN-alpha at doses corresponding to those used in clinical applications. If this previously demonstrated anhedonia is indeed part of a depression syndrome in rats, this behavioural symptom should be reversible by the administration of antidepressants. The objective of the present experiment was to determine whether two commonly used antidepressants (desipramine and fluoxetine) were effective in ameliorating IFN-alpha-induced anhedonia in rats. The experiment consisted of two phases. In the first, the effects of daily systemic injections of 104 units/kg of IFN-alpha (or vehicle) were evaluated with the three-bottle (1%, 8%, and 32%) sucrose-consumption test. In the second phase of the experiment, in addition to continued injections of IFN-, different groups received daily injections of desipramine (7.5 mg/kg ip), fluoxetine (7.5 mg/kg ip), or vehicle. The IFN-alpha injections during Phase 1 resulted in clear anhedonia, as expressed by increased consumption of the 32% solution and decreased consumption of 1% over the 33 days of this phase. After 15 days of antidepressant treatments, 32% sucrose consumption returned to baseline values. We have therefore confirmed that IFN-alpha-induced anhedonia is susceptible to reversal following chronic antidepressant treatment and thus it may appear timely to consider the prophylactic use of such in particular patients prescribed IFN in the clinic.

Interleukin-1 beta exerts a myriad of effects in the brain and in particular in the hippocampus: analysis of some of these actions

The realization, in the past decade or so, that bidirectional communication between the central nervous system and the immune system was likely has sparked an explosion of interest in the roles certain cytokines, particularly the proinflammatory cytokine interleukin-1 beta (IL-1 beta), might play in the brain. The observation that IL-1 type I receptor was expressed in highest density in the hypothalamus was of significance in identifying a role for IL-1 beta in neuroendocrine modulation. However, the finding that receptor expression was also high in the hippocampus, an area of the brain which plays a pivotal role in memory and learning, has led to uncovering a role for IL-1 beta in cognitive function. There is now a great deal of evidence suggesting that IL-1 beta plays a significant role in hippocampal synaptic function, and the possibility that IL-1 beta may trigger some of the detrimental changes in certain neurodegenerative diseases is currently being assessed. The review addresses some of the issues relating to the role of IL-1 beta in the brain, specifically in the hippocampus.

Cytokine-induced sickness behaviour: mechanisms and implications

Sickness behaviour represents the expression of the adaptive reorganization of the priorities of the host during an infectious episode. This process is triggered by pro-inflammatory cytokines produced by peripheral phagocytic cells in contact with invading micro-organisms. The peripheral immune message is relayed to the brain via a fast neural pathway and a slower humoral pathway, resulting in the expression of pro-inflammatory cytokines in macrophage-like cells and microglia in the brain. The cellular and molecular components of this previously unsuspected system are being progressively identified. These advances are opening new avenues for understanding brain disorders, including depression.

Time-dependent sensitization of corticotropin-releasing hormone, arginine vasopressin and c-fos immunoreactivity within the mouse brain in response to tumor necrosis factor-alpha

Stressor or cytokine treatments, such as interleukin-1beta, promote time-dependent alterations of hypothalamic-pituitary-adrenal functioning, including increased arginine vasopressin stores within corticotropin-releasing hormone (CRH) terminals in the external zone of the median eminence. Likewise, we have previously shown that the proinflammatory cytokine, tumor necrosis factor-alpha (TNF-alpha), provoked a time-dependent sensitization of neuroendocrine and brain monoamine activity. To further explore the protracted consequences of TNF-alpha, the present investigation determined whether the cytokine sensitized activity of neuroendocrine regulatory brain regions, as assessed by c-fos expression, and had protracted consequences on amygdaloid CRH, as well as hypothalamic corticotropin secretagogues. Indeed, immunoreactivity for arginine vasopressin and corticotropin-releasing hormone, and their colocalization within cell terminals of the median eminence, varied over time following an initial 4.0-microg tumor necrosis factor-alpha treatment, peaking after 7 days and normalizing within 28 days. Within the central amygdala, a sensitization effect was evident as reflected by increased CRH immunoreactivity, but this effect required re-exposure to the cytokine, unlike the median eminence changes that simply evolved with the passage of time. As well, tumor necrosis factor-alpha provoked a marked sensitization of c-fos staining within the paraventricular nucleus of the hypothalamus, supraoptic nucleus and the central amygdala. From these data we suggest that tumor necrosis factor-alpha influences responsivity of stressor-reactive brain regions and has protracted effects on central neuropeptide expression within the hypothalamus and central amygdala, although the time course for the effects vary across brain regions. Evidently, exposure to tumor necrosis factor-alpha may promote neuroplasticity of brain circuits involved in mediating neuroendocrine, sickness or inflammatory responses. It is suggested that such a sensitization may influence the response to immunological and traumatic insults and may thus be relevant to behavioral pathology.

Differential expression of various cytokine receptors in the brain after stimulation with LPS in young and old mice

The effect of lipopolysaccharides (LPS) on the expression of cytokine receptors was examined in the spleen, brain and pituitary gland, and compared in young and old mice. The level of mRNA for various cytokine receptors (IL-1RI, IL-2Ralpha, IL-3Ralpha, IL-6R, TNFalphaR and IFNgammaR) was found to be increased in the spleen of young but not in old mice within 2-6h of stimulation with LPS. Similar enhancement of cytokine receptor mRNA was also observed in the brain after LPS stimulation, but the magnitude varied according to the type of cytokine receptor, the site of brain and the age of the mice. In the hypothalamus, the level of mRNA for IL-1R, IL-3R, IL-6R and IFNgammaR increased in young but not in old mice. Reciprocally, in the cerebral cortex, mRNA for TNFalphaR and IFNgammaR increased in old but not in young mice. In the hippocampus, TNFalphaR mRNA expression, increased in young but not in old mice, and expression of the other cytokine receptors did not change greatly in either. In the pituitary gland, mRNA for IL-6R, TNFalphaR and IFNgammaR increased in both young and old mice, but IL-2Ralpha increased only in young mice.Thus, various cytokines produced by immune cells might directly or indirectly influence brain functions through the various cytokine receptors expressed in the brain. Moreover, interactions between the immune system and the brain at the time of infection would be expected to be different in young and old mice, because cytokine production changes with age, as does the expression of their receptors in the brain.

Activation of astrocyte intracellular signaling pathways by interleukin-1 in rat primary striatal cultures

The striatum has been implicated as the site of action mediating neurotoxic effects of interleukin-1 (IL-1) during ischemia. However, the molecular mechanisms underlying these events have yet to be fully addressed. In the present study, primary cultures of rat striatal cells were used as a model for the study of IL-1 signaling pathways in the striatum. Immunocytochemical analyses revealed that these cultures consisted of a mixture of neurones and astrocytes and demonstrated expression of the IL-1 type I receptor (IL-1RI) on both cell types. Treatment with IL-1 (3 units/ml) for 10 min increased phosphorylation of p38 MAP kinase in striatal cells. The endogenous IL-1RI inhibitor IL-1Ra (24 ng/ml) and the p38 MAP kinase inhibitor SB203580 (10 nM) both inhibited this response. Analysis of the effects of IL-1 on nuclear translocation of the transcription factor NF-kB revealed that NF-kB became activated in a time-dependent manner. Immunocytochemistry revealed that IL-1 stimulated p38 phosphorylation and NF-kB translocation in astrocytes only. TaqMan real-time quantitative PCR analysis revealed that IL-1 stimulated gene expression of tumor necrosis factor-alpha (TNF) in striatal cultures. The p38 MAP kinase inhibitor SB203580 failed to inhibit the effects of IL-1 on NF-kB translocation or gene transcription. These studies have demonstrated significant aspects of the IL-1 signaling cascade in cultured striatum. Of particular interest is the finding that IL-1 stimulated activation of p38 MAP kinase and NF-kB in striatal astrocytes exclusively.

IL-1 receptor type I gene expression in the amygdala of inflammatory susceptible Lewis and inflammatory resistant Fischer rats

Lewis (LEW/N) and Fischer (F344/N) rats have different responses to inflammatory and behavioral stressors due to differences in hypothalamus-pituitary-adrenal (HPA) axis function. For example, LEW/N rats are more sensitive to restraint, inflammation and experimentally induced autoimmunity due to decreased HPA activity. The HPA axis response to peripheral inflammation is mediated, at least in part, by IL-1beta and its receptor, IL-1 type I (IL-1RI). Here, we studied the distribution of IL-1RI mRNA in the brains of LEW/N and F344/N rats, and demonstrated that IL-1RI mRNA expression has significantly increased in the basolateral nucleus (BLA) of the amygdala of LEW/N rats. These findings suggest that strain-specific HPA axis responses may be mediated by extrahypothalamic pathways.

The central nucleus of the amygdala; a conduit for modulation of HPA axis responses to an immune challenge?

Physical stressors such as infection, inflammation and tissue injury elicit activation of the hypothalamic-pituitary-adrenal (HPA) axis. This response has significant implications for both immune and central nervous system function. Investigations in rats into the neural substrates responsible for HPA axis activation to an immune challenge have predominantly utilized an experimental paradigm involving the acute administration of the pro-inflammatory cytokine interleukin- 1β (IL-1β). It is well recognized that medial parvocellular corticotrophin-releasing factor cells of the paraventricular nucleus (mPVN CRF) are critical in generating HPA axis responses to an immune challenge but little is known about how peripheral immune signals can activate and/or modulate the mPVN CRF cells. Studies that have examined the afferent control of the mPVN CRF cell response to systemic IL-1β have centred largely on the inputs from brainstem catecholamine cells. However, other regulatory neuronal populations also merit attention and one such region is a component of the limbic system, the central nucleus of the amygdala (CeA). A large number of CeA cells are recruited following systemic IL-lβ administration and there is a significant body of work indicating that the CeA can influence HPA axis function. However, the contribution of the CeA to HPA axis responses to an immune challenge is only just beginning to be addressed. This review examines three aspects of HPA axis control by systemic IL-1β: (i) whether the CeA has a role in generating HPA axis responses to systemic IL-1β, (ii) the identity of the neural connections between the CeA and mPVN CRF cells that might be important to HPA axis responses and(iii) the mechanisms by which systemic IL-Iβ triggers the recruitment of CeA cells.

Interleukin-1 beta alters brown adipose tissue but not renal sympathetic nerve responses to hypothermia

Proinflammatory cytokines and acute physical stress influence sympathetic nerve discharge (SND). Because interleukin-1 beta (IL-1 beta) produces physiological responses that require central neural integration and because the sympathetic nervous system mediates physiological responses to environmental stress, we hypothesized that IL-1 beta modulates SND responses to acute physical stress. Therefore, this study examined the effects of IL-1 beta (290 ng/kg iv) and mild hypothermia on renal and interscapular brown adipose tissue (IBAT) SND regulation in chloralose-anesthetized rats. IBAT SND did not change after IL-1 beta administration but was significantly increased during acute mild hypothermia, which was induced 60 min after IL-1 beta treatment. Renal SND was unchanged after IL-1 beta administration and during hypothermia. Acute hypothermia, without prior IL-1 beta administration, did not alter IBAT and renal SND. Increases in IBAT SND during sustained (120 min) hypothermia were significantly higher in IL-1 beta-treated rats compared with saline-treated rats, whereas renal SND responses to sustained hypothermia did not differ among groups. Exposure to acute cold stress after sustained hypothermia produced greater increases in IBAT SND in IL-1 beta-treated rats compared with saline-treated controls. These data suggest that IL-1 beta alters IBAT SND responses to acute and sustained hypothermia.

How circulating cytokines trigger the neural circuits that control the hypothalamic-pituitary-adrenal axis

It is now no secret that the brain plays a crucial role in organizing, adapting and restraining the systemic inflammatory response via a complex cascade of mechanisms involving proteins of the innate immune system, molecules of the proinflammatory signal transduction pathways, prostaglandins (PGs) and specific populations of neurons. These neuronal circuits, in particular those controlling autonomic functions, are all together involved in engaging the physiological responses that may help eliminating the foreign material and adjust the inflammatory events to prevent detrimental consequences. For instance, elevation in plasma glucocorticoid levels is one of the most powerful endogenous and well-controlled feedback on the pro-inflammatory signal transduction machinery taking place across the organisms. The main Center that controls this neuroendocrine system is the paraventricular nucleus of the hypothalamus (PVN) that receives neuronal projections from numerous hypothalamic and extra-hypothalamic nuclei and areas. There is now compelling evidence that molecules produced by cells of the blood-brain barrier (BBB) may bind to their cognate receptors expressed at the surface of neurons that are responsible to trigger the hypothalamic-pituitary adrenal (HPA) axis. This review presents the new molecular insights regarding the pro-inflammatory signal transduction pathways that occur in these cells and how they are related to the neuroendocrine circuits mediating the increase in plasma glucocorticoid levels during systemic and localized immunogenic insults.

Characterization of the central nervous system innervation of the rat spleen using viral transneuronal tracing

Splenic immune function is modulated by sympathetic innervation, which in turn is controlled by inputs from supraspinal regions. In the present study, the characterization of central circuits involved in the control of splenic function was accomplished by injecting pseudorabies virus (PRV), a retrograde transynaptic tracer, into the spleen and conducting a temporal analysis of the progression of the infection from 60 hours to 110 hours postinoculation. In addition, central noradrenergic cell groups involved in splenic innervation were characterized by dual immunohistochemical detection of dopamine-beta-hydroxylase and PRV. Infection in the CNS first appeared in the spinal cord. Splenic sympathetic preganglionic neurons, identified in rats injected with Fluoro-Gold i.p. prior to PRV inoculation of the spleen, were located in T(3)-T(12) bilaterally; numerous infected interneurons were also found in the thoracic spinal cord (T(1)-T(13)). Infected neurons in the brain were first observed in the A5 region, ventromedial medulla, rostral ventrolateral medulla, paraventricular hypothalamic nucleus, Barrington's nucleus, and caudal raphe. At intermediate survival times, the number of infected cells increased in previously infected areas, and infected neurons also appeared in lateral hypothalamus, A7 region, locus coeruleus, subcoeruleus region, nucleus of the solitary tract, and C3 cell group. At longer postinoculation intervals, infected neurons were found in additional hypothalamic areas, Edinger-Westphal nucleus, periaqueductal gray, pedunculopontine tegmental nucleus, caudal ventrolateral medulla, and area postrema. These results demonstrate that the sympathetic outflow to the spleen is controlled by a complex multisynaptic pathway that involves several brainstem and forebrain nuclei.

Inhibitory action of nitric oxide on circulating tumor necrosis factor-induced NF-kappaB activity and COX-2 transcription in the endothelium of the brain capillaries

Circulating tumor necrosis factor alpha (TNF-alpha) has a profound stimulatory influence on mitogen-activated protein kinases that lead to nuclear factor kappa B (NF-kappaB) activity and transcription of the cyclooxygenase 2 (COX-2) gene in cells associated with the blood-brain barrier (BBB). This study investigated the hypothesis that nitric oxide (NO) acts as an endogenous modulator of TNF-induced NF-kappaB signaling and COX-2 transcription in the endothelium of the cerebral capillaries. To this end, rats were pretreated with the nonselective inhibitor of NO synthase (NOS) N(G)-nitro-L-arginine methyl ester (L-NAME) and killed 15, 45, and 90 minutes (min) after an i.v. injection of recombinant rat TNF-alpha. De novo expression of the inhibitory factor kappa B alpha (IkappaB alpha) was used as an index of NF-kappaB activity, whereas COX-2 mRNA induction was evaluated throughout the brain by in situ hybridization combined with immunohistochemistry. A single i.v. bolus of TNF caused a rapid expression of IkappaB alpha transcript first along large arterioles and small capillaries and thereafter within microglia across the brain parenchyma. The proinflammatory cytokine also provoked a strong transcriptional activation of the COX-2 gene that was quite specific to the cerebral endothelium as revealed by dual labeling using an antisera directed against the von Willebrand factor. Inhibition of NO synthesis did not by itself activate these proinflammatory molecules, but it enhanced the effects of circulating TNF-alpha in the BBB; the IkappaB alpha and COX-2 signal was significantly higher in microvascular-associated cells of animals that received both L-NAME and TNF-alpha treatments than those challenged with the proinflammatory cytokine alone. Rats treated with specific NOS inhibitors provided the evidence that these effects were mediated via the constitutive endothelial NOS (eNOS) and not the inducible form. These results indicate that eNOS-derived NO acts as an endogenous inhibitor of TNF-alpha-induced NF-kappaB activity and COX-2 transcription in the endothelium of the cerebral capillaries. This autoregulatory feedback of NO on these proinflammatory signal transduction events may be an essential element to prevent an exaggerated response that takes place in cells of the BBB during systemic immune challenges.

Effect of repeated lipopolysaccharide administration on tissue cytokine expression and hypothalamic-pituitary-adrenal axis activity in rats

The effects of chronic immune challenge on cytokine expression and hypothalamic-pituitary-adrenal axis (HPA) axis responses to stress were studied in Wistar rats after administration of increasing doses of lipopolysaccharide (LPS). Repeated LPS (R-LPS) decreased body weight and increased adrenal weight and pituitary pro-opiomelanocortin mRNA levels. LPS injection increased plasma adrenocorticotropic hormone (ACTH) and corticosterone but the effect was attenuated in R-LPS. Plasma corticosterone but not ACTH responses to restraint were also reduced in R-LPS. Basal and restraint-stimulated corticotropin releasing hormone (CRH) mRNA levels were lower in R-LPS, but responses to a new LPS injection were similar to controls. In contrast, type 1 CRH receptor (CRH-R1) mRNA responses to both LPS and restraint were blunted in R-LPS. Vasopressin mRNA levels in parvocellular neurones were higher in R-LPS, and increased further after restraint but not after a new LPS injection. Glucocorticoid receptor (GR) levels in the paraventricular nucleus (PVN) increased after a single LPS or R-LPS (24 h after the last injection) but declined after a new injection in R-LPS. Interleukin (IL)-1beta and IL-6 mRNAs increased in the pituitary, spleen and circumventricular organs after single or R-LPS, suggesting that cytokines may contribute to the activation of the HPA axis though pathways from the circumventricular organs as well as paracrine effects in the pituitary. The data show that (i) adaptation of the HPA axis during repeated LPS injection involves increases in vasopressin : CRH expression ratios in parvocellular neurones; (ii) that hypothalamic CRH and vasopressin responses to acute stimulation are independent of CRH-R1 expression in the PVN; and (iii) there is a dissociation between pituitary and adrenal responses to acute stress suggesting a decrease of adrenal sensitivity to ACTH.

Differential expression of orexin receptors 1 and 2 in the rat brain

Orexins (hypocretins) are neuropeptides synthesized in the central nervous system exclusively by neurons of the lateral hypothalamus. Orexin-containing neurons have widespread projections and have been implicated in complex physiological functions including feeding behavior, sleep states, neuroendocrine function, and autonomic control. Two orexin receptors (OX(1)R and OX(2)R) have been identified, with distinct expression patterns throughout the brain, but a systematic examination of orexin receptor expression in the brain has not appeared. We used in situ hybridization histochemistry to examine the patterns of expression of mRNA for both orexin receptors throughout the brain. OX(1)R mRNA was observed in many brain regions including the prefrontal and infralimbic cortex, hippocampus, paraventricular thalamic nucleus, ventromedial hypothalamic nucleus, dorsal raphe nucleus, and locus coeruleus. OX(2)R mRNA was prominent in a complementary distribution including the cerebral cortex, septal nuclei, hippocampus, medial thalamic groups, raphe nuclei, and many hypothalamic nuclei including the tuberomammillary nucleus, dorsomedial nucleus, paraventricular nucleus, and ventral premammillary nucleus. The differential distribution of orexin receptors is consistent with the proposed multifaceted roles of orexin in regulating homeostasis and may explain the unique role of the OX(2)R receptor in regulating sleep state stability.

How the immune and nervous systems interact during disease-associated anorexia

Anorexia is one of the most common symptoms associated with illness and constitutes an adaptive strategy in fighting acute infectious diseases. However, prolonged reduction in food intake and an increase in metabolic rate, as seen in the anorexia-cachexia syndrome, lead to depletion of body fat and protein reserves, thus worsening the organism's condition. Because the central nervous system controls many aspects of food intake, soluble factors known as cytokines that are secreted by immune cells might act on the brain to induce anorexia during disease. This review focuses on the communication pathways from the immune system to the brain that might mediate anorexia during disease. The vagus nerve is a rapid route of communication from the immune system to the brain, as subdiaphragmatic vagotomy attenuates the decrease in food-motivated behavior and c-Fos expression in the central nervous system in response to peripheral administration of the proinflammatory cytokine, interleukin-1beta, or bacterial lipopolysaccharide. At later time points after peripheral lipopolysaccharide administration, interleukin-1 itself acts in the brain to mediate anorexia and is found in the arcuate nucleus of the hypothalamus. The mechanisms by which interleukin-1beta gains access to the brain and the potential role of neuropeptide-Y-containing neurons in the arcuate hypothalamus in mediating anorexia during disease are discussed.

Effect of LPS administration on the expression of POMC, NPY, galanin, CART and MCH mRNAs in the rat hypothalamus

Anorexia and weight loss are manifestations of inflammation seen both in patients and in experimental animal models such as the lipopolysaccharide (LPS)-treated rat. Using in situ hybridization, the levels of mRNAs encoding proopiomelanocortin (POMC), neuropeptide Y (NPY), galanin, melanin-concentrating hormone (MCH) and cocaine- and amphetamine-regulated transcript (CART) were investigated in the rat hypothalamus after a single intraperitoneal dose (125 microg/kg) of LPS. Four hours after LPS injection the food intake was significantly decreased. POMC and CART mRNA levels were increased in the arcuate nucleus, and MCH, CART and galanin mRNAs were all decreased in the lateral hypothalamic area in LPS-treated rats. Levels of mRNAs for NPY and galanin in the arcuate nucleus, and for MCH and CART in the zona incerta did not change significantly after LPS treatment. These findings support the hypothesis that LPS-induced factors mediate signalling to the POMC/CART neurons in the arcuate nucleus which could lead to reduced food intake by decreasing MCH, CART and galanin synthesis in target lateral hypothalamic neurons.

Regulation of the gene encoding the monocyte chemoattractant protein 1 (MCP-1) in the mouse and rat brain in response to circulating LPS and proinflammatory cytokines

Accumulating evidence supports the existence of an innate immune response in the brain during systemic inflammation that is associated with a robust induction of proinflammatory cytokines and chemokines by specific cells of the central nervous system. The present study investigated the genetic regulation and fine cellular distribution of the monocyte chemoattractant protein-1 (MCP-1) in the brain of mice and rats in response to systemic immune insults. MCP-1 belongs to a superfamily of chemokines that have a leading role in the early chemotaxic events during inflammation. In situ hybridization histochemistry failed to detect constitutive expression of the chemokine transcript in the cerebral tissue except for the area postrema (AP) that exhibited a low signal under basal conditions. This contrasts with the strong and transient induction of the mRNA encoding MCP-1 following a single systemic bolus of lipopolysaccharide (LPS), recombinant interleukin-1 beta (IL-1 beta) and tumor necrosis factor alpha (TNF-alpha). These stimuli rapidly triggered (30 to 90 minutes) MCP-1 transcription in all the circumventricular organs (CVOs), the choroid plexus (chp), the leptomeninges, and along the cerebral blood vessels. The time-related induction and intensity of the signal differed among the challenges, route of administration and species, but MCP-1-expressing cells were always found in vascular-associated structures and those devoid of blood-brain barrier. At later times, few isolated microglia across the brain parenchyma depicted positive signal for MCP-1 mRNA. A dual-labeling procedure also provided convincing anatomical evidence that endothelial cells of the microvasculature and a few myeloid cells of the CVOs and chp were positive for the transcript during endotoxemia. This gene is under a sophisticated transcriptional regulation, as the hybridization signal returned to undetectable levels 12 to 24 hours after all the treatments in both species. Of interest are the data that only ligands that triggered nuclear factor kappa B (NF-kappa B) signaling had the ability to increase MCP-1 gene expression, because high doses of IL-6 remained without effects. These data provide the anatomical evidence that MCP-1 is expressed within specific populations of cells in response to systemic inflammatory molecules that use NF-kappa B as intracellular signaling system. This chemokine may therefore play a critical role in the cerebral innate immune response and contribute to the early chemotaxic events during chronic cerebral inflammation.

Cardiovascular responses to subseptic doses of endotoxin contribute to differential neuronal activation in rat brain

The contribution of cardiovascular activity in the early central responses to systemic inflammation was assessed in rats following intravenous administration of subseptic doses of lipopolysaccharide (LPS). LPS at 12.5 microg/kg increased heart rate (HR) but did not alter mean arterial pressure (MAP), and induced interleukin-1 beta (IL-1 beta) gene expression at 1 h in circumventricular organs (CVOs), choroid plexus, meninges, blood vessels, and pituitary gland. IL-1 beta mRNA levels were attenuated at 2 h in most regions studied. LPS at 50 microg/kg caused a biphasic change in MAP, increased HR, increased levels of arginine vasopressin heteronuclear RNA in the hypothalamic paraventricular nucleus (PVN), and induced IL-1 beta gene expression in the nucleus of the solitary tract (NTS) at 1 h. LPS (both doses) induced Fos-like immunoreactivity (FLI) in the area postrema, organum vasculosum of the lamina terminalis, NTS, preoptic area, supraoptic nucleus, and PVN at 1 h. In the PVN, neurons with FLI were found primarily in the dorsal and dorsal medial parvocellular divisions after 12.5 microg/kg of LPS whereas neurons with FLI were found throughout the PVN after 50 microg/kg of LPS. After 2 h, FLI was widespread throughout the brain. Plasma ACTH levels were elevated at 1 and 2 h in response to both doses of LPS, and levels of CRF mRNA were increased after 2 h in the parvocellular PVN. Our results reveal that central responses to increasing doses of LPS show different patterns which are related to activation of distinct immune and viscerosensory pathways, and that cardiovascular responses contribute to early neuronal activation as LPS concentrations are increased.

Diffusion and action of intracerebroventricularly injected interleukin-1 in the CNS

Interleukin-1beta acts on the CNS to induce fever, neuroendocrine activation and behavioural depression. We have previously demonstrated that interleukin-1beta is synthesized in glial cells and macrophages of circumventricular organs and choroid plexus after intraperitoneal administration of bacterial lipopolysaccharide. Whether, and how, interleukin-1beta produced in glial cells affects neuronal functioning is unknown. Diffusion throughout the extracellular space is an important pathway by which factors produced by glial cells act on distant cells, a phenomenon coined "volume transmission". The present study assessed diffusion of recombinant rat interleukin-1beta, recombinant human interleukin-1 receptor antagonist and 10mol. wt dexran in the rat CNS after intracerebroventricular administration to model interleukin-1beta release from choroid plexus. Immunocytochemistry with specific antibodies directed against interleukin-1beta and interleukin-1 receptor antagonist revealed that these molecules rapidly penetrated into periventricular tissue and spread along white matter fibre bundles and blood vessels in the caudoputamen, hypothalamus and amygdala. The transcription factor nuclear factor kappa B and the immediate-early gene product Fos were detected immunocytochemically to reveal interleukin-1beta action. Intracerebroventricular infusion of interleukin-1beta induced nuclear factor kappa B translocation in choroid plexus, ependymal cells, basolateral amygdala, cerebral vasculature and meninges. Fos immunoreactivity was found in the supraoptic and paraventricular hypothalamus and central amygdala. We propose that intracerebroventricular injected interleukin-1beta can enter the brain parenchyma and act as a "volume transmission" signal in, for example, the basolateral amygdala where it might activate a neuronal projection to the central amygdala.

Cytokines regulate expression of the type 1 interleukin-1 receptor in rat hippocampal neurons and glia

Interleukin-1 beta is a key mediator of inflammation and stress in the central nervous system (CNS). This cytokine induces CNS glial cells to produce numerous additional cytokines and growth factors under inflammatory conditions. We have investigated regulation of the signal transducing type 1 interleukin-1 receptor (IL-1R1) in the CNS. In vivo, IL-1R1 was not detected in glial cells under basal conditions but was strongly induced after a stab lesion. Cultured astrocytes were used to identify specific signals that regulate expression of the receptor. IL-1R1 mRNA and protein were induced by inflammatory stimuli including tumor necrosis factor (TNF alpha) and IL-1 beta itself. Although expression of the receptor was not detected in glia under basal conditions in vivo, pyramidal neurons in the hippocampus expressed the IL-1 receptor in the normal, unlesioned brain. Cultured embryonic hippocampal neurons were used to investigate specific stimuli that regulate IL-1R1 in neurons. As in astrocytes, IL-1 and TNF alpha induced expression of IL-1R1. The expression of IL-1R1 in hippocampal neurons suggests a possible role for IL-1 in regulating neuronal function, and indicates that these neurons may be directly influenced by cytokines.

Choroid plexus: target for polypeptides and site of their synthesis

Choroid plexus (CP) is an important target organ for polypeptides. The fenestrated phenotype of choroidal endothelium facilitates the penetration of blood-borne polypeptides across the capillary walls. Thus, both circulating and cerebrospinal fluid (CSF)-borne polypeptides can reach their receptors on choroidal epithelium. Several polypeptides have been demonstrated to regulate CSF formation by controlling blood flow to choroid plexus and/or the activity of ion transport in choroidal epithelium. However, many ligand-receptor interactions occurring in the CP are not involved in the regulation of fluid secretion. Increasing evidence suggests that the choroidal epithelium plays an important role in hormonal signaling via a receptor-mediated transport into the brain (e.g., leptin) and helps to clear certain CSF-borne polypeptides (e.g., soluble amyloid beta-protein). Thus, impaired choroidal transport or insufficient clearance of polypeptides may contribute to pathogenesis of systemic or central nervous system (CNS) disorders, such as obesity or Alzheimer's disease. CP epithelium is not only a target but is also a source of neuropeptides, growth factors, and cytokines in the CNS. These polypeptides following their release into the CSF may exert distal, endocrine-like effects on target cells in the brain due to bulk flow of this fluid. Distinct temporal patterns of choroidal expression of several polypeptides are observed during brain development and in various CNS disorders, including traumatic brain injury and ischemia. Therefore, it is proposed that the CP plays an integral role not only in normal brain functioning, but also in the recovery from the injury. This review attempts to critically analyze the available data to support the above hypothesis.

Subdiaphragmatic vagotomy blocks interleukin-1beta-induced fever but does not reduce IL-1beta levels in the circulation

Peripheral interleukin-1beta has been implicated in the initiation of fever responses, yet the pathways by which it influences brain function are still unclear. Sectioning the abdominal vagus has been reported to inhibit fever after intraperitoneal administration of interleukin-1beta, suggesting that vagal afferents participate in signaling the brain to mount a fever response to interleukin-1beta. However, the inhibitory effect of subdiaphragmatic vagotomy could be due to alterations in pharmacokinetics such that the intraperitoneally injected cytokine does not reach the general circulation in sufficient quantities to activate the brain via blood-borne signaling. We measured both fever and plasma levels of interleukin-1beta in vagotomized and sham-operated rats after intraperitoneal administration of 1 microg/kg human recombinant interleukin-1beta to determine whether vagotomy reduces fever and levels of circulating interleukin-1beta after intraperitoneal injection. Plasma levels of human recombinant and endogenous rat interleukin-1beta were measured in separate enzyme-linked immunosorbent assays. While intraperitoneal administration of human recombinant interleukin-1beta elevated plasma levels of this cytokine similarly in vagotomized and sham-operated animals, only sham-operated rats responded with fever. Plasma levels of endogenous rat interleukin-1beta were unchanged by any treatment. These results demonstrate that the blockade of intraperitoneal interleukin-1beta-induced fever after subdiaphragmatic vagotomy cannot be accounted for by alterations of interleukin-1beta levels in the general circulation.