Regulation of interleukin-1 receptors and hypothalamic-pituitary-adrenal axis by lipopolysaccharide treatment in the mouse

LPS-induced inflammation potentiates the IL-1β-mediated reduction of LH secretion from the anterior pituitary explants

Acting at the level of the brain, interleukin- (IL-)1 β is considered to be one of the most potent downregulators of reproduction processes during immune/inflammatory challenge. IL-1 β suppresses gonadotropin-releasing hormone (GnRH) secretion from the hypothalamus resulting in the inhibition of the luteinizing hormone (LH) release from the anterior pituitary (AP). However, the presence of IL-1 β receptors in the AP suggests the possible direct action of this cytokine on LH secretion. The study was designed to determine the effect of IL-1 β on the LH secretion from the AP explants collected from saline and LPS-treated ewes in the follicular phase. It was found that IL-1 β suppressed (P ≤ 0.01) GnRH-stimulated LH release and LH β gene expression in AP explants in both groups. However, IL-1 β action was more potent in the explants collected from LPS-treated animals. Pituitaries from LPS-treated animals were characterized by increased (P ≤ 0.01) IL-1 type I receptor and decreased (P ≤ 0.01) GnRH receptor gene expression level compared to the saline-treated group. IL-1 β also affected the GnRH-R gene expression in explants collected from LPS-treated animals. Our results show that direct action of IL-1 β on the pituitary gonadotropes could be one of the reasons of the reproductive processes disorders accompanying an inflammatory state.

Changes in interleukin-1 signal modulators induced by 3,4-methylenedioxymethamphetamine (MDMA): regulation by CB2 receptors and implications for neurotoxicity

BACKGROUND

3,4-Methylenedioxymethamphetamine (MDMA) produces a neuroinflammatory reaction in rat brain characterized by an increase in interleukin-1 beta (IL-1β) and microglial activation. The CB2 receptor agonist JWH-015 reduces both these changes and partially protects against MDMA-induced neurotoxicity. We have examined MDMA-induced changes in IL-1 receptor antagonist (IL-1ra) levels and IL-1 receptor type I (IL-1RI) expression and the effects of JWH-015. The cellular location of IL-1β and IL-1RI was also examined. MDMA-treated animals were given the soluble form of IL-1RI (sIL-1RI) and neurotoxic effects examined.

METHODS

Dark Agouti rats received MDMA (12.5 mg/kg, i.p.) and levels of IL-1ra and expression of IL-1RI measured 1 h, 3 h or 6 h later. JWH-015 (2.4 mg/kg, i.p.) was injected 48 h, 24 h and 0.5 h before MDMA and IL-1ra and IL-1RI measured. For localization studies, animals were sacrificed 1 h or 3 h following MDMA and stained for IL-1β or IL-1RI in combination with neuronal and microglial markers. sIL-1RI (3 μg/animal; i.c.v.) was administered 5 min before MDMA and 3 h later. 5-HT transporter density was determined 7 days after MDMA injection.

RESULTS

MDMA produced an increase in IL-ra levels and a decrease in IL-1RI expression in hypothalamus which was prevented by CB2 receptor activation. IL-1RI expression was localized on neuronal cell bodies while IL-1β expression was observed in microglial cells following MDMA. sIL-1RI potentiated MDMA-induced neurotoxicity. MDMA also increased IgG immunostaining indicating that blood brain-barrier permeability was compromised.

CONCLUSIONS

In summary, MDMA produces changes in IL-1 signal modulators which are modified by CB2 receptor activation. These results indicate that IL-1β may play a partial role in MDMA-induced neurotoxicity.

Modulation of interleukin-1 receptors followed by endotoxin lipopolysaccharide treatment in the mouse AtT-20 pituitary tumor cell line

OBJECTIVE

We have previously reported the characterization and regulation of interleukin-1 (IL-1) receptors utilizing [125I]IL-1 binding assay in male C57BL/6 mice and the mouse AtT-20 pituitary tumor cells. In the present study, we examine IL-1 receptors using an immunoblotting method to further characterize the mechanisms regulating the interactions of IL-1 receptors with endotoxin, lipopolysaccharide (LPS).

METHODS

We established Western blotting for IL-1 receptors using AtT-20 mouse pituitary tumor cells.

RESULTS

Several bands were seen; however, only the 105-kD band was neutralized with a 5-fold excess of IL-1 receptor- blocking peptides, suggesting that this band is specific for IL-1 receptors. Next, we investigated the effect of LPS and IL-1beta on IL-1 receptors. Treatment of AtT-20 cells with 0.01 microg/ml of LPS did not affect IL-1 receptors. In contrast, 1 microg/ml of LPS significantly increased IL-1 receptors in AtT-20 cells compared with the control group. In addition, [125I]IL-1beta binding was markedly increased followed by 1 microg/ml of LPS. In contrast, 1 nM recombinant human IL-1beta significantly decreased IL-1 receptors in AtT-20 cells compared with the control group although treatment of AtT-20 cells with 0.01 nM IL-1beta did not affect IL-1 receptors. LPS (0.1 and 1 microg/ml) did not affect IL-1beta concentrations in the medium of AtT-20 cell culture. IL-1beta concentrations in the homogenates from AtT-20 cells were significantly decreased after 1 microg/ml of LPS treatment but not after 0.01 microg/ml LPS.

CONCLUSIONS

These data demonstrate that LPS and IL-1beta differentially modulate IL-1 receptors in AtT-20 cells and LPS-induced modulation of IL-1 receptors may provide a novel mechanism for the actions of LPS to alter pituitary function during endotoxemia. Additional in vivo studies are necessary to determine the physiological relevance of this in vitro phenomenon.

Impaired adrenocorticotropic hormone response to bacterial endotoxin in mice deficient in prostaglandin E receptor EP1 and EP3 subtypes

Sickness evokes various neural responses, one of which is activation of the hypothalamo-pituitary-adrenal (HPA) axis. This response can be induced experimentally by injection of bacterial lipopolysaccharide (LPS) or inflammatory cytokines such as IL-1. Although prostaglandins (PGs) long have been implicated in LPS-induced HPA axis activation, the mechanism downstream of PGs remained unsettled. By using mice lacking each of the four PGE receptors (EP1-EP4) and an EP1-selective antagonist, ONO-8713, we showed that both EP1 and EP3 are required for adrenocorticotropic hormone release in response to LPS. Analysis of c-Fos expression as a marker for neuronal activity indicated that both EP1 and EP3 contribute to activation of neurons in the paraventricular nucleus of the hypothalamus (PVN). This analysis also revealed that EP1, but not EP3, is involved in LPS-induced activation of the central nucleus of the amygdala. EP1 immunostaining in the PVN revealed its localization at synapses on corticotropin-releasing hormone-containing neurons. These findings suggest that EP1- and EP3-mediated neuronal pathways converge at corticotropin-releasing hormone-containing neurons in the PVN to induce HPA axis activation upon sickness.

Cytokines and neuro-immune-endocrine interactions: a role for the hypothalamic-pituitary-adrenal revolving axis

Cytokines, peptide hormones and neurotransmitters, as well as their receptors/ligands, are endogenous to the brain, endocrine and immune systems. These shared ligands and receptors are used as a common chemical language for communication within and between the immune and neuroendocrine systems. Such communication suggests an immunoregulatory role for the brain and a sensory function for the immune system. Interplay between the immune, nervous and endocrine systems is most commonly associated with the pronounced effects of stress on immunity. The hypothalamic-pituitary-adrenal (HPA) axis is the key player in stress responses; it is well established that both external and internal stressors activate the HPA axis. Cytokines are chemical messengers that stimulate the HPA axis when the body is under stress or experiencing an infection. This review discusses current knowledge of cytokine signaling pathways in neuro-immune-endocrine interactions as viewed through the triplet HPA axis. In addition, we elaborate on HPA/cytokine interactions in oxidative stress within the context of nuclear factor-kappaB transcriptional regulation and the role of oxidative markers and related gaseous transmitters.

Effect of non-peptide corticotropin-releasing factor receptor type 1 antagonist on adrenocorticotropic hormone release and interleukin-1 receptors followed by stress

Previous studies demonstrated that ether-laparotomy significantly increased iodine-125-labeled interleukin-1alpha ([125I]IL-1alpha) binding in the mouse anterior pituitary at 2 h after the onset of stress. Corticotropin-releasing factor (CRF) receptor antagonist, D-Phe CRF (12-41), abolished ether-laparotomy-induced increase in [125I]IL-1alpha binding in the pituitary, showing that CRF plays a pivotal role in the regulation of IL-1 receptors under stress conditions. In an attempt to define the effect of CRA 1000 (2-(N-(2-methylthio-4-isopropylphenyl)-N-ethylamino-4-(4-(3-fluorophenyl)-1,2,3,6-tetrahydropyridin-1-yl)-6-methylpyrimidine), a non-peptide CRF receptor type 1 antagonist on the regulation of hypothalamic-pituitary-adrenal (HPA) axis and IL-1 receptors in the mouse, we measured plasma adrenocorticotropic hormone (ACTH) and corticosterone levels, [125I]IL-1alpha binding and the expression of transcripts for type 1 IL-1 receptor (IL-1R1 mRNA) in the pituitary at 2 h after endotoxin lipopolysaccharide (LPS) treatment or ether-laparotomy stress with or without CRA 1000 pretreatment. A single injection of LPS dramatically increased plasma ACTH and corticosterone levels compared with saline injection. In contrast, plasma ACTH levels were significantly attenuated in response to one LPS injection following oral CRA 1000 pretreatment. LPS-induced plasma corticosterone levels tended to be lower after CRA 1000 pretreatment but it did not reach statistical significance. Ether-laparotomy stress significantly increased plasma ACTH and corticosterone levels at 2 h after the onset of stress and CRA 1000 pretreatment did not affect the peak ACTH and corticosterone levels following stress. Ether-laparotomy stress resulted in a robust increase in [125I]IL-1alpha binding and IL-1R1 mRNA levels in the pituitary. CRA 1000 pretreatment significantly decreased ether-laparotomy stress-induced IL-1R1 mRNA levels but did not affect [125I]IL-1alpha binding. Pretreatment with CRA 1000 without stress significantly increased [125I]IL-1alpha binding and IL-1R1 mRNA levels compared with those in vehicle pretreatment. These data demonstrate differential effects of CRA 1000 in HPA axis following endotoxin and ether-laparotomy stress and complex interactions between CRF and IL-1 receptors during stress.

Locomotor activity changes following lipopolysaccharide treatment in mice: a multivariate assessment of behavioral tolerance

To determine the effects of repeated, acute endotoxin exposure on locomotor behavior, male laboratory mice were injected intraperitoneally with lipopolysaccharide (LPS: 50, 100 or 200 microg/kg) or saline vehicle on experimental Days 1, 4 and 7. At 2 h after each treatment, locomotor activity was assessed in a nonnovel, automated open-field apparatus (Digiscan) for 30 min. On Day 1, all horizontal and vertical activity measures were significantly reduced to near zero values by each dose of LPS. Behavioral tolerance to LPS formed rapidly, as locomotor activity of the treated groups did not differ from the control group on Days 4 or 7. In a second study, mice were given LPS (50, 100 or 150 microg/kg ip) or saline vehicle on two test days, 28 days apart. Activity was assessed, 1 h after injection, in a novel open field on the first test day and in a nonnovel open field on the second test day. Significant locomotor activity decrements were readily apparent in LPS-treated mice only in the nonnovel open field. This latter finding indicates that environmental novelty mediates, at least partially, the locomotor-reducing effects of LPS in mice.

Modulation of type I interleukin-1 receptor messenger RNA followed by one and repeated endotoxin treatment in the mouse

The expression of transcripts for type 1 interleukin-1 receptors (IL-1R1) were investigated utilizing reverse transcription-polymerase chain reaction (RT-PCR) techniques in male C57BL/6 mice. First, we measured IL-1R1 mRNA 1, 2, 6, and 12 h after a single injection of endotoxin lipopolysaccharide (LPS; 30 microg/mouse). IL-1R1 mRNA was measured in relevant tissues utilizing semiquantitative RT-PCR. IL-1R1 mRNA levels were significantly increased 2 and 6 h after one LPS injection group in comparison with the saline injection group in the hippocampus and testis. Next, we investigated the effect of repeated injections of LPS on IL-1R1 expression. LPS or saline were injected at 24-h intervals for 4 consecutive days. The mice were divided into four groups: (1) LPS injections for 4 days and LPS injection on day 5 (LPS-LPS); (2) LPS injections for 4 days and saline injection on day 5 (LPS-saline); (3) saline injections for 4 days and LPS injection on day 5 (saline-LPS); (4) saline injections for 4 days and saline injection on day 5 (saline-saline). The mice were sacrificed by decapitation 2 h after the last injection. In the hippocampus and pituitary gland, significant increases of IL-1R1 mRNA levels were observed in the saline-LPS group, however, the increases were attenuated in LPS-LPS group. IL-1R1 mRNA levels in the hypothalamus were unchanged in all four groups. In the adrenal gland and testis, IL-1R1 mRNA levels were significantly increased in the LPS-LPS group as well as saline-LPS group. These data demonstrate differential regulation of IL-1R1 mRNA by one and repeated endotoxin treatment and suggest the importance of the cytokines in mediating the effects of infectious challenge on brain, endocrine and immune function during endotoxemia.

Regulation of the hypothalamic-pituitary-adrenal axis by cytokines: actions and mechanisms of action

Glucocorticoids are hormone products of the adrenal gland, which have long been recognized to have a profound impact on immunologic processes. The communication between immune and neuroendocrine systems is, however, bidirectional. The endocrine and immune systems share a common "chemical language," with both systems possessing ligands and receptors of "classical" hormones and immunoregulatory mediators. Studies in the early to mid 1980s demonstrated that monocyte-derived or recombinant interleukin-1 (IL-1) causes secretion of hormones of the hypothalamic-pituitary-adrenal (HPA) axis, establishing that immunoregulators, known as cytokines, play a pivotal role in this bidirectional communication between the immune and neuroendocrine systems. The subsequent 10-15 years have witnessed demonstrations that numerous members of several cytokine families increase the secretory activity of the HPA axis. Because this neuroendocrine action of cytokines is mediated primarily at the level of the central nervous system, studies investigating the mechanisms of HPA activation produced by cytokines take on a more broad significance, with findings relevant to the more fundamental question of how cytokines signal the brain. This article reviews published findings that have documented which cytokines have been shown to influence hormone secretion from the HPA axis, determined under what physiological/pathophysiological circumstances endogenous cytokines regulate HPA axis activity, established the possible sites of cytokine action on HPA axis hormone secretion, and identified the potential neuroanatomic and pharmacological mechanisms by which cytokines signal the neuroendocrine hypothalamus.

Interleukin-1beta system (ligand, receptor type I, receptor accessory protein and receptor antagonist), TNF-alpha, TGF-beta1 and neuropeptide Y mRNAs in specific brain regions during bacterial LPS-induced anorexia

Bacterial lipopolysaccharide (LPS) or endotoxin induces neurological manifestations including anorexia. It is proposed that LPS-induced cytokine production is involved in the generation of neurological manifestations and in neuroinflammatory/immunological responses during gram-negative infections. For example, LPS-induced effects can be blocked or ameliorated by the interleukin-1 receptor antagonist (IL-1Ra). Here, sensitive and specific RNase protection assays were used to investigate the effects of the intracerebroventricular (i.c.v.) administration of LPS on mRNA levels of interleukin-1beta (IL-1beta) system components, tumor necrosis factor (TNF)-alpha, transforming growth factor (TGF)-beta1, and neuropeptide Y (NPY) in the cerebellum, hippocampus, and hypothalamus. The same brain region sample was analyzed with all of the antisense probes. The data show simultaneous local induction of multiple cytokine components messenger ribonucleic acids (mRNAs) within specific brain regions in anorectic rats responding to i.c.v. administered LPS (500 ng/rat). Interleukin-1beta and IL-1Ra had a similar mRNA induction profile (hypothalamus > cerebellum > hippocampus). Interleukin-1 receptor type I (IL-1RI) mRNA also increased in all three brain regions examined, and the soluble form of IL-1 receptor accessory protein (IL-1R AcP II) mRNA was induced in the hypothalamus. Tumor necrosis factor-alpha mRNA levels increased in the hypothalamus > hippocampus > cerebellum. Levels of membrane bound IL-1R AcP, TGF-beta1, and NPY mRNAs did not change significantly in any brain region. The results suggest that: (1) endogenous up-regulation of IL-1beta and TNF-alpha in the hypothalamus contribute to LPS-induced anorexia; and (2) the ratio IL-1Ra/IL-1beta, and IL-1beta <--> TNF-alpha interactions may have implications for gram-negative infections associated with high levels of LPS in the brain-cerebrospinal fluid.

Endotoxin induced increases in rat plasma pituitary-adrenocortical hormones are better reflected by alterations in tumor necrosis factor alpha than interleukin-1beta

In order to assess the relative cytokine contribution to endotoxin stimulation of pituitary-adrenocortical hormone secretion, we measured plasma levels of interleukin-1beta (IL-1beta), tumor necrosis factor (TNF alpha), adrenocorticotropin (ACTH) and corticosterone following lipopolysaccharide (LPS) challenge in rats. LPS administration induced robust increases in both plasma ACTH and corticosterone levels at 3 h after i.p. injection; while ACTH decreased towards control levels, corticosterone remained at peak concentrations at 6 h after LPS injection. Basal levels of plasma IL-1beta were below the sensitivity of the ELISA and basal levels of plasma TNF alpha were 0.25+/-0.12 pM. Small but highly variable non-significant increases in plasma IL-1beta levels were seen at 3 h and 6 h after injection of LPS. The lack of functional consequences of the small increases in IL-1beta levels was demonstrated by unchanged levels of [125I]IL-1alpha binding in liver at 3 h after LPS injection. In contrast, dramatic increases in plasma TNF alpha concentrations were observed at 3 h and decreased to non-injected control levels at 6 h after LPS injection. There was a significant positive correlation between ACTH and TNF alpha after LPS injection, while no correlation was seen between ACTH and IL-1beta. These data demonstrate differential regulation of IL-1beta and TNF alpha by endotoxin treatment and suggest that TNF alpha may be a more potent mediator of LPS-induced ACTH secretion in rat.

Neuroimmunomodulation via limbic structures--the neuroanatomy of psychoimmunology

During the last 20 years, mutual communications between the immune, the endocrine and the nervous systems have been defined on the basis of physiological, cellular, and molecular data. Nevertheless, a major problem in the new discipline "Psychoneuroimmunology" is that controversial data and differences in the interpretation of the results make it difficult to obtain a comprehensive overview of the implications of immunoneuroendocrine interactions in the maintenance of physiological homeostasis, as well as in the initiation and the course of pathological conditions within these systems. In this article, we will first discuss the afferent pathways by which immune cells may affect CNS functions and, conversely, how neural tissues can influence the peripheral immune response. We will then review recent data, which emphasize the (patho)physiological roles of hippocampal-amygdala structures and the nucleus accumbens in neuroimmunomodulation. Neuronal activity within the hippocampal formation, the amygdaloid body, and the ventral parts of the basal ganglia has been examined most thoroughly in studies on neuroendocrine, autonomic and cognitive functions, or at the level of emotional and psychomotor behaviors. The interplay of these limbic structures with components of the immune system and vice versa, however, is still less defined. We will attempt to review and discuss this area of research taking into account recent evidences for neuroendocrine immunoregulation via limbic neuronal systems, as well as the influence of cytokines on synaptic transmission, neuronal growth and survival in these brain regions. Finally, the role of limbic structures in stress responses and conditioning of immune reactivity will be commented. Based on these data, we propose new directions of future research.

Rat homolog of mouse interleukin-1 receptor accessory protein: cloning, localization and modulation studies

A protein which facilitates the binding between interleukin-1 (IL-1) and the type I IL-1 receptor (designated as interleukin-1 receptor accessory protein, IL-1RAcP) has recently been cloned in mouse cells. In the present study, a rat homolog of the mouse IL-1RAcP was isolated from a rat superior cervical ganglion library. The deduced 570 amino acid sequences between rat and mouse IL-1RAcP have > 95% sequence identity to each other with similar predicted signal peptide sequence (20 amino acids), extracellular domain (339 amino acids), a single transmembrane domain (24 amino acids) and a long intracellular domain (187 amino acids). The rat IL-1RAcP has approximately 25% sequence identity to the rat type I IL-1 receptor and a predicted extracellular domain with three immunoglobulin-like loops. RNase protection assays demonstrated that rat IL-1RAcP is expressed in both brain and peripheral tissues with the highest densities present in liver and brain areas such as hypothalamus, cerebral cortex, hippocampus and cerebellum; significantly lower densities were present in lung and in immune tissues such as thymus and spleen. The presence of IL-1RAcP in brain was confirmed by in situ hybridization histochemical studies with a discrete localization present in the dentate gyrus of the hippocampus. The IL-1RAcp was down-regulated in parallel with the type I IL-1 receptor in the liver following endotoxin treatment in rats. These data demonstrating the presence and modulation of a rat homolog of a mouse IL-1RAcP, which is highly expressed in brain and peripheral tissues containing type I rat IL-1 receptor, further suggest the importance of the interaction between the two proteins in rat in modulating the actions of IL-1. On the other hand, the presence of the IL-1RAcP in brain areas which show an absence of type I IL-1 receptors suggests additional functions for this protein in the rat.

Interleukin-1 receptors type I and type II in the mouse brain: kinetics of mRNA expressions after peripheral administration of bacterial lipopolysaccharide

The expression of transcripts for Interleukin-1 (IL-1) type I and type II receptors (IL-1R1, IL-1R2) was investigated in the mouse brain and spleen using reverse transcription-polymerase chain reaction techniques under basal conditions and following injection of endotoxin (LPS, i.p., 4 mg/kg). Under basal conditions, mRNAs for both receptor types were found in various parts of the brain, in pituitary as well as in spleen. Following LPS stimulation, mRNA expressions were increased in all studied tissues. IL-1R1 mRNAs were predominant in the brain and pituitary while, IL-1R2 mRNAs were more abundant in the spleen. The maximal quantity of transcripts (IL-1R1, IL-1R2) was obtained 6 h after LPS injection in all studied tissues. The decrease to basal level was observed within 48 h in the brain. In the spleen, IL-1R1 mRNAs remained elevated 48 h after LPS while IL-1R2 mRNAs had already reached basal level. These results indicate a LPS-induced stimulation of IL-1 receptors mRNAs in the brain and a differential expression of IL-1R1 and IL-1R2 transcripts in brain and immune tissues.

Interleukin-1 receptors in the brain-endocrine-immune axis. Modulation by stress and infection

In the present study, we examined the in vitro and in vivo modulation of IL-1 receptors by stress and endotoxin treatment. The treatment of AtT-20 mouse pituitary adenoma cells for 24 h with neuroendocrine mediators of stress such as CRF and catecholamines produced dose-dependent increases in cAMP production and [125I]IL-1 alpha binding. In contrast, somatostatin and dexamethasone significantly inhibited CRF-stimulated cAMP production and decreased both basal and CRF-mediated increases in [125I]IL-1 alpha binding. Furthermore, in keeping with the effects of stress mediators to up-regulate IL-1 receptors in AtT-20 cells, ether-laparotomy stress in mice resulted in a significant increase in [125I]IL-1 alpha binding in the pituitary with no significant alterations observed in the brain; in contrast, [125I]oCRF binding in the pituitary was significantly decreased after the ether-laparotomy stress. Next, we investigated the modulation of IL-1 beta levels and [125I]IL-1 alpha binding following endotoxin treatment. IL-1 beta levels were dramatically increased in the peripheral tissues (pituitary, testis, and spleen) at 2-6 h after a single LPS injection (30 micrograms LPS/mouse); however, no significant changes were observed in brain (hippocampus and hypothalamus). [125I]IL-1 alpha binding in the pituitary gland, liver, spleen, and testis was significantly decreased at 2 h following a single administration of both low (30 micrograms LPS/mouse) and high (300 micrograms LPS/mouse) doses of endotoxin. [125I]IL-1 alpha binding in the hippocampus was not significantly altered at 2 h by low dose of LPS and was significantly decreased by high-dose administration of LPS (300 micrograms/mouse). Following two LPS injections (at 0 and 12 h), dramatic increases in IL-1 beta concentrations in the hypothalamus, hippocampus, spleen, and testis were observed at 2 h after the second LPS injection; a small but statistically nonsignificant change was evident in the pituitary. Moreover, dramatic decreases in [125I]IL-1 alpha binding were seen after two injections of 30 micrograms LPS/mouse in both central and peripheral tissues. These data provide further support for a role for IL-1 in coordinating brain-endocrine-immune responses to stress and infection.

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

Systemic interleukin-1 (IL-1) activates the hypothalamo-pituitary-adrenal (HPA) axis, an effect exerted through increased synthesis and secretion of corticotropin-releasing factor (CRF) by parvicellular neurosecretory neurons. The site(s) and mechanism(s) through which circulating IL-1 may access central systems governing HPA axis output remain obscure. To identify potential cellular targets for blood-borne IL-1, we analyzed the distribution of mRNA encoding the rat type 1 IL-1 receptor (IL-1R1) in rat brain. Regional ribonuclease protection assays detected a single protected fragment corresponding to the membrane-bound form of the IL-1R1 mRNA in all areas analyzed. In situ hybridization revealed labeling predominantly over barrier-related cells, including the leptomeninges, non-tanycytic portions of the ependyma, the choroid plexus, and vascular endothelium. Low to moderate levels of the IL-1R1 mRNA were detected in just a few neuronal cell groups, including the basolateral nucleus of the amygdala, the arcuate nucleus of the hypothalamus, the trigeminal and hypoglossal motor nuclei, and the area postrema. No specific labeling for IL-1R1 mRNA was detected over neurons that respond to intravenous IL-1 beta by induction of transcription factor Fos, including hypophysiotropic CRF cells and brainstem catecholamine neurons. Injection of IL-1 beta did, however, provoke induction of mRNA encoding the immediate-early gene, NGFI-B, but not c-fos, in two major loci of IL-1R1 expression, vascular endothelial cells, and the area postrema. Intravenous injection of IL-1 beta acutely down-regulated IL-1R1 mRNA in perivascular cells, but not in neuronal cell groups. These results suggest the parenchymal sites of IL-1R1 expression in rat to be distinct from those reported previously in mouse. The common expression in both species of an IL-1R in non-neuronal elements highlights the possibility that IL-1-mediated activation of CRF neurons may result from cytokine-receptor interaction at vascular, and/or other barrier-related, sites to trigger release of secondary signalling molecules in a position to interact with components of HPA control circuitry.

Modulation of interleukin-1 receptors in the neuro-endocrine-immune axis

Interleukin-1 (IL-1) receptors with kinetics, pharmacological and biochemical characteristics of type I IL-1 receptors have been identified in the mouse neuro-endocrine-immune axis. In the present study, we examined the in-vitro and in-vivo modulation of IL-1 receptors by stress and endotoxin treatment. The treatment of AtT-20 mouse pituitary adenoma cells for 24 hr with neuro-endocrine mediators of stress such as corticotropin releasing factor (CRF) and catecholamine (beta 2 adrenergic) receptor agonists produced a dose-dependent increase in cAMP and [125I]IL-1 alpha binding. In contrast, somatostatin and dexamethasone significantly inhibited CRF-stimulated cAMP production and decreased both basal and CRF-mediated increase of [125I]IL-1 alpha binding. Furthermore, in keeping with the effects of stress mediators to upregulate IL-1 receptors in AtT-20 cells, ether-laparotomy stress in mice resulted in a significant increase in [125I]IL-1 alpha binding in the pituitary with no significant alterations observed in the brain; in contrast, [125I]oCRF binding in the pituitary was significantly decreased after the ether-laparotomy stress. Next, we investigated the modulation of IL-1 beta levels and [125I]IL-1 alpha binding following endotoxin lipopolysaccharide (LPS) treatment. IL-1 beta levels were dramatically increased in the peripheral tissues (pituitary, testis and spleen) at 2-6 hr after a single LPS injection (30 micrograms LPS/mouse). However, no significant changes were observed in brain (hippocampus and hypothalamus). [125I]IL-1 alpha binding in the pituitary gland, liver, spleen and testis was significantly decreased at 2 hr following a single administration of both low (30 micrograms LPS/mouse) and high (300 micrograms LPS/mouse) doses of endotoxin. [125I]IL-1 alpha binding in the hippocampus was not significantly altered at 2 hr by a low dose of LPS and was significantly decreased by high dose administration of LPS (300 micrograms/mouse). Following two LPS injections (at 0 and 12 hr), dramatic increases in IL-1 beta concentrations in the hypothalamus, hippocampus, spleen and testis were observed at 2 hr after the second LPS injection; a small but statistically nonsignificant change was evident in the pituitary. Moreover, dramatic decreases in [125I]IL-1 alpha binding were seen after two injections of 30 micrograms LPS/mouse in both central and peripheral tissues. These data provide further support for a role for IL-1 in co-ordinating neuro-endocrine-immune responses to stress and infection.