Activation of the hypothalamus-pituitary-adrenal axis by bacterial endotoxins: routes and intermediate signals

Brain neurotransmission during peripheral inflammation

It is now well established that an inflammatory challenge as evoked by bacterial endotoxin (LPS) induces autonomic, endocrine, and behavioral responses that are controlled by the brain. However, detailed information on the neuronal pathways and neurotransmitters involved is scarce. We used in vivo microdialysis and biotelemetry in rats to monitor hippocampal and preoptic serotonergic and noradrenergic neurotransmission, body temperature, and heart rate after an i.p. LPS injection. Moreover, free corticosterone levels were measured in the dialysates, and behavioral activity was scored by visual observation. Apart from a biphasic fever response, tachycardia, elevated free corticosterone levels, and sickness behavior, peripheral injection of LPS caused a dramatic increase in preoptic extracellular concentrations of noradrenaline, but no effect on serotonin in this structure. The increase in preoptic noradrenaline levels appears to underlie the first fever phase and may participate in hypothalamic-pituitary-adrenocorticul axis activation. In contrast, whereas LPS had only a moderate effect on hippocampal noradrenaline, a marked increase in hippocampal extracellular serotonin levels was found. Use of the interleukin (IL)-1 receptor antagonist and the cyclooxygenase inhibitor indomethacine learned that IL-1 and prostaglandins are mediators in this response. Our data show that an endotoxin challenge results in highly differentiated changes in brain neurotransmission, probably subserving the coordinate processing of immune information in circuits involved in autonomic, neuroendocrine, and behavioral regulation.

Acute and subacute effects of endotoxin on hypothalamic gaseous neuromodulators

Although two-way communication between the hypothalamus and the immune system in now well established, particularly for the hypothalamo-pituitary-adrenal axis, the role of the gaseous neurotransmitters nitric oxide (NO) and carbon monoxide (CO) is much less well understood in terms of hypothalamic function. These agents are an important part of the peripheral inflammatory response; and their synthetic enzymes, NO synthase (NOS) and heme oxygenase (HO), respectively, have been localized to the hypothalamic PVN and SON. The induced generation of both NO and CO leads to the suppression of CRH and vasopressin, the major stimulators of the HPA. Thus, the addition of hemin to hypothalamic explants is maximally active at 1 microM in attenuating the release of CRH and vasopressin, and this dose is also most effective in generating biliverdin and associated CO. CO generation is also able to stimulate cyclooxygenase to produce prostaglandin E2, an established intermediary in the cytokine-stimulated activation of the HPA. Finally, inducible NOS mRNA is specifically induced in the hypothalalmus in response to endotoxin, in parallel to interleukin-1. These data provide increasing evidence in favor of NO and CO as counterregulatory agents in the HPA response to immune activation.

Stress, neuropsychiatric disorders and immunological effects exerted by benzodiazepines

Psychoneuroimmunology is a growing scientific field which deals with the mutual interplay between nervous and immune systems. In this framework, many data have demonstrated that cytokines (CKs) derived from the periphery are able to cross the blood brain barrier and act upon the central nervous system (CNS) [e.g., the hypothalamic-pituitary-adrenal axis (HPAA)], thus regulating several physiological functions (thermoregulation, sleep, appetite) or damaging the nervous tissue, when released in exaggerated amounts. On the other hand, nervous cells, such as astrocytes and microglial cells also generate proinflammatory CKs which may be detrimental for the CNS. The neuromodulating CK network can be triggered by microorganisms and/or their products (i.e. bacterial endotoxins), but also stressful life events may activate the HPAA, thus affecting the immune system function. This review will place emphasis on some clinical conditions, such as phobia and migraine without aura (MWA), characterized by anxiety disorders. Patients affected by these neuropsychiatric alterations exhibit multiple functional deficits of phagocytes and T lymphocytes which allow penetration of various pathogens into the host. This is also supported by the detection of circulating bacterial endotoxins and the evidence of both spontaneous and induced exaggerated release of proinflammatory CKs in phobic and MWA patients. The possible iatrogenic effects of benzodiazepines (BDZ) on the immune system have been evaluated by in vitro and in vivo studies. In this respect, it emerges that diazepam exerts an inhibitory function on the immune system, while alprazolam behaves as an immunoenhancer. The presence of central and/or peripheral BDZ receptors on immune cells seems to be the key mechanism responsible for the immunomodulation exerted by these drugs.

Effects of prenatal alcohol and pair feeding on lipopolysaccharide-induced secretion of TNF-alpha and corticosterone

Fetal alcohol exposure (FAE) produces profound alterations in immunological and neuroendocrine functions. The present study examined the effects of FAE on the secretion of tumor necrosis factor (TNF-alpha) and corticosterone following administration of lipopolysaccharide (LPS) in normal (N) adult rats, in adult offspring of dams fed a liquid diet supplemented with ethanol (E), and in pair-fed control offspring (P). LPS-induced TNF-alpha secretion was not affected by either gender or prenatal treatment. In contrast, LPS-induced corticosterone secretion was significantly greater in female than in male rats, and at 60-min post-LPS was significantly higher in E and P, compared to N females. Ovariectomy significantly inhibited LPS-induced TNF-alpha secretion in E, but not in P and N, rats and chronic replacement with 17-beta-estradiol markedly inhibited TNF-alpha secretion in ovariectomized E and N, but not in P, rats. In contrast, ovariectomy reduced the effects of LPS on corticosterone secretion in all groups, and chronic replacement with 17-beta-estradiol reversed this effect. These findings indicate that LPS-induced secretion of corticosterone, but not TNF-alpha, is affected by prenatal manipulations and by gender. In addition, alterations in the hormonal environment in females modulate LPS-induced corticosterone secretion in all prenatal treatment groups, but differentially influence TNF-alpha secretion in rats exposed to alcohol, restricted feeding, or normal diets in utero.

Inflammatory disease as chronic stress

It is now established that communication between the CNS and the immune system is bidirectional, that endocrine factors can alter immune function and that immune responses can alter both endocrine and CNS responses. In many respects CNS and endocrine responses to acute inflammation are similar to the changes associated with acute stress exposure. In contrast, during chronic inflammation associated with adjuvant induced arthritis (AA), although circulating levels of corticosterone are increased, the peptidergic regulation of the hypothalamus is different from that seen during acute stress. As the disease progresses, a paradoxical reduction occurs in CRH mRNA in the paraventricular nucleus (PVN), whereas PVN AVP mRNA increases. These data suggest that there is increased expression of AVP mRNA within the CRH cells of the PVN with an increased emphasis on AVP regulation of HPA output. Additionally, HPA function is altered during chronic inflammation such that responses to psychological stress (i.e. restraint) are significantly dampened, while responses to further inflammatory challenges are maintained. These data suggest that alterations in PVN peptide colocalization may be important in regulating the progression of peripheral inflammatory responses and that the effects of inflammation on the hypothalamus alter stress-responsive systems. In addition to the AA model, we have similarly observed alterations in PVN peptide mRNA expression with disease onset in the murine MRL lpr/lpr and MRL +/+ model of SLE. Disease onset in murine SLE is spontaneous and does not rely on exogenous application of adjuvant; however, decreased levels of CRH in the PVN were observed from early disease onset in this animal model. It is suggested that alterations in CRH regulation in response to either acute or chronic inflammation may contribute as etiological factors to both psychiatric (i.e. neuropsychiatric SLE) and stress-related disease.

Mechanisms of hypothalamic-pituitary-adrenal axis stimulation by immune signals in the adult rat

Immune stimulation increases the activity of the HPA axis, a phenomenon directly or indirectly mediated through cytokines. We have used two models, the peripheral administration of endotoxin (LPS) or turpentine-induced tissue injury to show that corticotropin-releasing factor (CRF) and vasopressin (VP), hypothalamic peptides released by cytokines, play a dominant role in the increased ACTH measured in these two paradigms. In turn, CRF and VP synthesis and/or release is modulated by catecholamines, prostaglandins (PGs), and nitric oxide (NO). These secretagogues are produced in the periphery and/or the central nervous system (CNS) in response to increased cytokine levels and act on CRF/VP neurons and nerve terminals. Finally, endotoxemia and local tissue inflammation may upregulate brain levels of tumor necrosis factor alpha, interleukin-1 beta, and/or interleukin-6, providing yet another mechanism through which the occurrence of systemic inflammation is conveyed to the brain. The relative importance of brain or peripheral intermediates appears to depend on the site at which cytokine levels are increased. We have shown, for example, that peripheral, but not brain, PGs are important in mediating the neuroendocrine influence of blood-borne cytokines, while PGs in the CNS play a role in situations characterized by elevated brain immune proteins. NO, on the other hand, restrains the response of the HPA axis to circulating, but not brain cytokines. These results illustrate the complexity of the mechanisms involved in the stimulation of the HPA axis and suggest that their specific involvement depends on the type, intensity, and duration of immune stimulation.

Interleukin-1-induced plasticity of hypothalamic CRH neurons and long-term stress hyperresponsiveness

Infections and endotoxin (LPS) can affect hypothalamic CRH neurons and activate the HPA system. This can be prevented by IL-1 receptor antagonist and mimicked by IL-1. Chronic activation of the HPA system by repeated or chronic administration of IL-1 (1 week) to rats is associated with plastic changes in hypothalamic CRH neurons. Single administration IL-1 beta (5 micrograms/kg i.p.) to male Wistar or Lewis rats induced a similar form of neuroplasticity 1-3 weeks later. This is characterized by a selective increase in coproduction, costorage, and cosecretion of AVP in hypothalamic CRH neurons. Exposure of IL-1-primed rats 1-2 weeks later to foot shocks or IL-1 resulted in exaggerated ACTH and CORT responses as compared to vehicle-primed controls. Thus, rats are hyperresponsive to stressors weeks after IL-1 exposure. In IL-1-primed animals, CRH binding and CRH- and V1b receptor mRNA levels in the pituitary glands are not altered by IL-1 exposure 2 weeks earlier. We conclude that IL-1-induced, long-lasting hyperresponsiveness to stressors is primarily caused by functional alterations in the brain that may be directly related to observed plasticity of hypothalamic CRH neurons.

Effects of peripheral administration of LPS on the expression of immunoreactive interleukin-1 alpha, beta, and receptor antagonist in rat brain

The cytokine interleukin-1 (IL-1) appears to play a pivotal role in the orchestration of brain-mediated, nonspecific illness symptoms during an infection. In the present study, we examine the possibility that IL-1 is produced in the central nervous system itself, which may be responsible for the induction of brain-mediated responses. Using immunocytochemical techniques, we demonstrated that peripheral administration of bacterial endotoxin to rats caused a time- (1.5-24 hr) and dose-dependent (4 micrograms/kg-2.5 mg/kg) induction of IL 1 beta immunoreactivity in cells identified as macrophages in meninges and choroid plexus and microglial cells in various brain regions. At 8 hr after endotoxin (2.5 mg/kg), immunoreactive IL-1 alpha was observed in the same areas and cell types as IL-1 beta. Although no quantitative measurements have been performed, it appears that fewer cells express immunoreactive IL-1 alpha than IL-1 beta. Furthermore, IL-1ra was found to be constitutively expressed in neurons in the paraventricular nucleus and supraoptic nucleus, which is in accordance with mRNA data. After administration of endotoxin, we observed no additional cells that expressed immunoreactive IL-1ra. We conclude that IL-1 alpha and IL-1 beta production in the brain is induced in the same cell types, whereas IL-1ra is expressed constitutively by a different cell type--probably neurons.

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.

Depression, stress and immunological activation: the role of cytokines in depressive disorders

Traditionally, both stress and depression have been associated with impaired immune function and increased susceptibility to infectious and neoplastic disease. However over the last number of years a large body of evidence suggests that major depression is associated with signs of immunological activation. Moreover it has been suggested that cytokine hypersecretion may be involved in the aetiology of depressive disorders. The present article reviews the evidence from both clinical and experimental studies which implicates immunological activation and particularly hypersecretion of cytokines in the onset and maintenance of depressive illness. Both clinical and experimental studies indicate that stress and depression are associated with increased circulating concentrations of cytokines such as IL-1beta, IL-6 and gamma-IFN and positive acute phase proteins, and hyperactivity of the HPA-axis. In addition, it has been reported that immunological activation induces "stress-like" behavioural and neurochemical changes in laboratory animals. Although for many years it has been suggested that stress acts a predisposing factor to depressive illness, the precise mechanisms by which stress-induced depressive symptoms occur are not fully understood. Nevertheless, behavioural changes due to stress have often been explained in terms of changes in neurotransmitter function in the brain. In the present article increased cytokine secretion is implicated as a mechanism whereby stress can induce depression.

Endotoxin induces interleukin-1beta and nitric oxide synthase mRNA in rat hypothalamus and pituitary

The gases nitric oxide (NO) and carbon monoxide (CO) may be involved in hypothalamo-pituitary-adrenal axis (HPA) modulation. In the brain, NO is synthesized by two forms of NO synthase (NOS), a constitutive neuronal form (nNOS) and an inducible form (iNOS). There are also a constitutive heme oxygenase (HO2) and an inducible form (HO1) which generate CO. We have therefore investigated the effect of peripheral lipopolysaccharide (LPS) administration on the gene expression of these enzymes along with interleukin-1beta (IL-1beta) gene expression in the hypothalamus, pituitary and liver. Male Wistar rats (200-250 g body weight) were injected intraperitoneally with endotoxin (Escherichia coli, 055 B5) dissolved in sterile normal saline [250 microg/kg first group, 2.5 mg/kg (second group) and 6.25 mg/kg (third group)] in a final volume of 0.5 ml, or saline alone in the control group. The first and the second groups were studied 1, 3, 8 and 24 h after LPS (n = 4 per group); the third group was studied at 3 h. Total RNA was extracted from the hypothalamus, pituitary and liver, and cDNA was made using standard reverse transcriptase methods. Duplex polymerase chain reaction (PCR) was standardised in order to quantify the expression of a specific gene in relation to the 'house-keeping' gene beta-actin. The specific genes studied were iNOS, nNOS, HO1, HO2 and IL-1beta. The PCR products were separated on agarose gel and densitometric analysis of the bands allowed semi-quantification. In the second group, iNOS and IL-1beta were induced in hypothalamus, pituitary and liver, showing a peak at 3 h (p < 0.001), returning to baseline levels at 24 h. Neuronal NOS was not expressed in the liver under basal conditions or after LPS; in the hypothalamus and pituitary, nNOS was expressed basally but there was no change after LPS. In the first group, iNOS and IL-1beta were again induced in all three tissues studied, but with a delayed time course compared to the second and third groups; the peak change for IL-1beta occurred at 8 h (p < 0.05), again returning to baseline levels at 24 h. The peak for iNOS occurred at 24 h. HO1 and HO2 were expressed in all three tissues under basal conditions; HO1 was increased at 1 h in the liver in the second group, and at 3 h in the pituitary in the third group. There was no change in either HO1 or HO2 in the hypothalamus at any dose at any time point. We conclude that IL-1beta and iNOS are induced in rat hypothalamus and pituitary following various doses of endotoxin. We speculate that while IL-1beta may mediate stimulation of the HPA by endotoxin, NO generation may be involved in the counter-regulation of this response.

Regulation of uridine diphosphate glucuronosyltransferase during the acute-phase response

The acute-phase response is associated with profound effects on oxidative drug metabolism. However, the effects on glucuronidation are poorly characterized. The aim of the present study was to determine the role of mediators of the acute-phase response in the regulation of hepatic uridine diphosphate glucuronosyltransferase (UGT) expression. Family 1 and family 2 UGT isoforms were studied in turpentine-injected rats and in primary hepatocyte cultures exposed to cytokines and/or dexamethasone. In the in vivo model, glucuronidation of p-nitrophenol was unaffected, while testosterone glucuronidation was reduced to 65% of control (P<0.01). In contrast, the mRNA level of UGT1*1 (which metabolizes bilirubin, not phenols) was depressed to 16% of control (P<0.002), while the mRNA level of UGT2B3 (which metabolizes testosterone) was reduced to 53% (P<0.05). In primary hepatocyte culture, dexamethasone treatment resulted in a 3.4-fold induction of UGT1*1 mRNA levels (P<0.001) but only a 1.5-fold induction of UGT2B3 (P=0.1). Interleukin-6 in the presence of dexamethasone resulted in a marked dose-dependent suppression of both UGT1*1 and UGT2B3, although to different degrees. Interleukin-1 had no effect on UGT mRNA levels. Thus, inflammatory mediators, such as cytokines and glucocorticoids, may be important determinants of both oxidative and conjugative drug metabolism by the liver.

Transport of CRH from mouse brain directly affects peripheral production of beta-endorphin by the spleen

The blood-brain barrier (BBB) regulates the passage of substances between the brain and the periphery. It has not been shown that the secretion from the brain of a small amount of a substance can directly affect the periphery by transport across the BBB. We found that central injection of radioactively labeled corticotropin-releasing hormone (CRH) resulted in the accumulation of intact CRH in the spleen. CRH also increased splenic beta-endorphin, an effect not blocked by pretreatment with dexamethasone. Inhibition of the secretion of CRH from the brain by colchicine resulted in decreased accumulation of CRH in the spleen and also decreased splenic beta-endorphin. Similar findings occurred in the pituitary gland. The results show that the passage of labeled CRH from the brain can directly affect a peripheral organ, thus emphasizing the regulatory function of the BBB.

Role of circulating endotoxin and interleukin-6 in the ACTH and corticosterone response to intraperitoneal LPS

Peripheral administration of lipopolysaccharide (LPS) may activate the hypothalamus-pituitary-adrenal (HPA) axis by way of both neural and humoral mechanisms. We have investigated whether biologically active endotoxin appears in the general circulation after intraperitoneal administration of LPS (5 or 100 micrograms/kg) to rats and whether this is a prerequisite for activation of this HPA axis. Within 15 min, endotoxin appeared in the general circulation, whereas elevations of plasma adrenocorticotropic hormone (ACTH), corticosterone, and interleukin (IL)-6 concentrations were not detected until 90 min after LPS injection. At this time, a marked interindividual variation was observed in plasma concentrations of endotoxin, ACTH, corticosterone, and IL-6. Elevated levels of plasma endotoxin were associated with elevated levels of ACTH, corticosterone, and IL-6. Intravenous administration of the LPS antagonist cationic antimicrobial protein 18 (5 mg/kg), which did not affect cytokine production in the peritoneal cavity, markedly reduced plasma ACTH, corticosterone, and IL-6 levels after 5 micrograms/kg LPS. Our results suggest that circulating endotoxin is required for the activation of the HPA axis. They also favor a role for circulating IL-6 in this response.

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.

Induction of nitric oxide synthase and interleukin-1beta, but not heme oxygenase, messenger RNA in rat brain following peripheral administration of endotoxin

Previous studies have suggested that both nitric oxide (NO) and carbon monoxide (CO) are important modulators of the inflammatory response, while more recent data have implicated both gases as regulators of hypothalamic neuroendocrine function, particularly the hypothalamo-pituitary-adrenal axis. We have, therefore, investigated the modulation of the transcripts for the synthetic enzymes for both NO and CO following the intraperitoneal administration of lipopolysaccharide, serotype B5 055, over the course of 24 h. The mRNA for type I or neuronal nitric oxide synthase (nNOS), and type II or inducible (iNOS), and heme oxygenase1 ('inducible') and heme oxygenase2 ('constitutive'), were reverse transcribed to cDNA, amplified by the polymerase chain reaction, and then quantified using a co-amplified internal standard, beta-actin. This allowed for assessment of relative changes in transcript concentration. In addition, these were compared to changes in expression of the cytokine, IL-1beta. Finally, absolute levels of the synthetic enzyme transcripts were assessed by means of co-amplification in the presence of varying amounts of mutant templates in a competitive PCR reaction. Our data revealed rapid induction of IL-1beta, iNOS and HO1 in the liver, returning to baseline at 24 h. In the hypothalamus, all transcripts were present under basal conditions, but only IL-1beta and iNOS were induced by the LPS. We conclude that hypothalamic IL-1beta and iNOS can be induced by a non-lethal dose of endotoxin, and are, thus, in a position to mediate certain of the neuroendocrine consequences to inflammatory stress.

Evidence for an intramedullary prostaglandin-dependent mechanism in the activation of stress-related neuroendocrine circuitry by intravenous interleukin-1

We have provided evidence that the stimulatory effects of intravenous interleukin-1 (IL-1) on neurosecretory neurons in the paraventricular nucleus (PVH) that express corticotropin-releasing factor (CRF) depend specifically on the integrity of catecholaminergic projections originating in caudal medulla. Here we report on experiments designed to test alternative means by which circulating IL-1 might access medullary aminergic neurons, including mechanisms involving sensory components of the vagus, the area postrema, or perivascular cells bearing IL-1 receptors. Neither abdominal vagotomy nor area postrema lesions reliably altered Fos expression induced in the medulla or PVH in response to a moderately suprathreshold dose of IL-1beta. Cytokine-stimulated increases in CRF mRNA in the PVH were also unaffected by either ablation. By contrast, systemic administration of the cyclooxygenase inhibitor indomethacin resulted in parallel dose-related attenuations of IL-1 effects in hypothalamus and medulla. Microinjections of prostaglandin E2 (PGE2; >/=10 ng) in rostral ventrolateral medulla, the principal seat of IL-1-sensitive neurons that project to the PVH, provoked discrete patterns of cellular activation in hypothalamus and medulla that mimicked those seen in response to intravenous IL-1. We interpret these findings as supporting the hypothesis that paracrine effects of PGE2 released from perivascular cells in the medulla as a consequence of IL-1 stimulation and, acting through prostanoid receptors on or near local aminergic neurons that project to the PVH, contribute to the stimulatory effects of increased circulating IL-1 on neurons constituting the central limb of the hypothalamo-pituitary-adrenal axis.

Linomide activates the adrenocortical axis in the rat: inhibition of experimental autoimmune encephalomyelitis by linomide is not related to the increase of corticosterone

Linomide is a synthetic compound that affects various immunological functions and inhibits experimental autoimmune encephalomyelitis (EAE). In the present study we evaluated the effect of linomide on the HPA axis functions under basal and stress-induced conditions and examined whether the effect of linomide on the HPA axis is involved in linomide-induced amelioration of EAE in rats. Linomide caused a significant increase of serum ACTH and corticosterone (CS). The adrenocortical response to various stress modalities as well as the negative feedback exerted by glucocorticoids was not affected. The marked reduction of thymus weight following linomide treatment was abrogated in adrenalectomized rats. The induction of EAE in adrenalectomized rats was completely inhibited by linomide treatment. These results suggest that the increased CS levels induced by linomide are responsible for the decrease in thymus weight but do not play a role in the therapeutic effect of this drug in EAE.

Cytokine secretion in squirrel monkeys

The squirrel monkey, a non-human New World primate, has several endocrine peculiarities, including a 10-fold higher plasma cortisol concentration than Old World primates, such as man. Glucocorticoids are known to have immunomodulatory properties. We therefore measured cytokine levels in supernatants of in vitro cultures of mononuclear cells from the peripheral blood of squirrel monkeys and humans. We stimulated monocytes and lymphocytes with lipopolysaccharide (LPS) and phytohemagglutinin (PHA) in the presence or absence of hydrocortisone. Squirrel monkey monocytes secreted a more than 100-fold lower level of interleukin-1 beta (IL-1 beta) but a four-fold higher level of transforming growth factor beta (TGF-beta) than human monocytes, whereas the secretion of other cytokines, such as tumor necrosis factor alpha (TNF-alpha), TNF-beta and interleukin 2 (IL-2), did not differ between squirrel monkeys and humans. However, in squirrel monkey lymphocytes, the PHA-stimulated secretion of TNF-alpha was much greater than that of TNF-beta. Our results support the view that in squirrel monkeys there are subtle adaptations in some immune functions, particularly linked to the hypothalamic-pituitary-adrenal (HPA) system rather than a global suppression of the immune system.

Coincidental changes in behavior and plasma cortisol in unrestrained pigs after intracerebroventricular injection of tumor necrosis factor-alpha

The coincidental behavioral and physiological responses to inflammatory stimuli administered either peripherally or centrally were evaluated. In the first study, twenty castrated male pigs were injected ip with 0, 0.5, 5, or 50 microg/kg BW lipopolysaccharide (LPS). Body temperature was monitored telemetrically, and serial blood samples were collected via an indwelling jugular catheter for determination of plasma cortisol and tumor necrosis factor-alpha (TNF-alpha) concentrations. Sickness behaviors were measured during 10-min tests at 0, 2, 4, 8, 12, and 24 h post injection. The 5 and 50 microg/kg doses of LPS increased plasma concentrations of cortisol and TNF-alpha, while inducing anorexia, hypersomnia, and fever. In contrast, although 0.5 microg/kg LPS induced acute anorexia, hypersomnia, and fever, it did not increase plasma TNF-alpha; and the cortisol response was small and transient, suggesting the behavioral system in pigs is more responsive to LPS than the hypothalamic-pituitary-adrenal (HPA) axis. Because LPS-induced behavior and activation of the HPA axis involve proinflammatory cytokines in the brain, in a second study, unrestrained pigs with jugular catheters were injected intracerebroventricularly (I.C.V.) with recombinant porcine TNF-alpha. Vehicle or TNF-alpha (0, 5, or 50 ng/kg) was injected I.C.V., and plasma cortisol and behavior were determined as before. Pigs injected I.C.V. with 50 ng/kg TNF-alpha showed anorexia, hypersomnia, and an abrupt increase in plasma cortisol concentration. Whereas 5 ng/kg TNF-alpha I.C.V. also induced marked sickness behavior, it failed to stimulate the HPA axis, as indicated by plasma cortisol levels. That there was a distinct difference in the magnitude of behavioral and endocrine responses to LPS and TNF-alpha suggests that different systems that are responsive to inflammatory stimuli exhibit different sensitivities.

Defect in interleukin-1beta secretion prevents sickness behavior in C3H/HeJ mice

To examine the role of interleukin-1beta (IL-1beta) in mediating sickness, we studied the effects of lipopolysaccharide (LPS) and IL-1beta on social behavior in endotoxin-responsive C3H/HeOuJ (OuJ) mice and endotoxin-resistant C3H/HeJ (HeJ) mice. Whereas LPS (1, 10 and 100 microg) depressed social behavior and body weight compared to saline in OuJ mice, in HeJ mice it did not. To determine if the refractoriness of HeJ mice to the behavioral effects of LPS was related to secretion of IL-1beta, in a second study, HeJ and OuJ mice were injected IP with LPS (10 microg) and plasma concentration of IL-1beta was determined postinjection. At 4 h postinjection, the plasma concentration of IL-1beta was increased by LPS in OuJ mice, but not in HeJ mice. The increase in plasma IL-1beta in OuJ mice corresponded to the maximal depression in social behavior. To further verify that HeJ mice are refractory to the behavioral effects of LPS because they fail to respond and produce cytokines, the social behavior of HeJ and OuJ mice injected IP with recombinant murine IL-1beta (0, 50, 100, or 200 ng) was compared. As anticipated, exogenous IL-1beta depressed social behavior similarly in endotoxin-responsive OuJ mice and endotoxin-resistant HeJ mice. These data indicate that a genetic mutation in HeJ mice that prevents LPS-induced synthesis of cytokines also renders HeJ mice refractory to the behavioral effects of LPS.

Inhibition of tumor necrosis factor-alpha action within the CNS markedly reduces the plasma adrenocorticotropin response to peripheral local inflammation in rats

The present study tested the hypothesis that the cytokine tumor necrosis factor-alpha (TNF-alpha) is an important CNS mediator of the hypothalamo-pituitary-adrenal (HPA) axis response to local inflammation in the rat. Recombinant murine TNF-alpha administered directly into the cerebroventricles of normal rats produced a dose-dependent increase in plasma adrenocorticotropin (ACTH) concentration. Local inflammation induced by the intramuscular injection of turpentine (50 microl/100 gm body weight) also produced an increase in plasma ACTH, peaking at 160-200 pg/ml at 7.5 hr after injection (compared with 10-30 pg/ml in controls). Intracerebroventricular pretreatment with either 5 microl of anti-TNF-alpha antiserum or 1-50 microg of soluble TNF receptor construct (rhTNFR:Fc) reduced the peak of the ACTH response to local inflammation by 62-72%. In contrast, intravenous treatment with the same doses of anti-TNF-alpha or rhTNFR:Fc had no significant effect on the ACTH response to local inflammation. Although these data indicated an action of TNF-alpha specifically within the brain, no increase in brain TNF-alpha protein (measured by bioassay) or mRNA (assessed using either in situ hybridization histochemical or semi-quantitative RT-PCR procedures) was demonstrable during the onset or peak of HPA activation produced by local inflammation. Furthermore, increased passage of TNF-alpha from blood to brain seems unlikely, because inflammation did not affect plasma TNF-alpha biological activity. Collectively these data demonstrate that TNF-alpha action within the brain is critical to the elaboration of the HPA axis response to local inflammation in the rat, but they indicate that increases in cerebral TNF-alpha synthesis are not a necessary accompaniment.

Corticosterone responses of adult Lewis and Fischer rats

Indwelling catheters were implanted into the inferior vena cava of adult male and female Lewis/N and Fischer 344 rats. Each animal was exposed to ACTH, novelty stimulation, nicotine, lipopolysaccharide (LPS), and saline on 5 consecutive days. Blood was withdrawn before (baseline) and at several time points after the stimulus on each day. There were no differences in baseline corticosterone levels nor in responses to saline in any group. In general, responses to stimulation peaked at 15-30 min and returned to baseline by 60-90 min. Corticosterone responses to LPS showed a different time course; maximal responses occur at 1-2 h and return to baseline by 24 h. Fischer animals showed higher corticosterone levels than Lewis rats during the response to stimulation, but returned to baseline at the same times. Females of each strain showed higher corticosterone responses than males at 15, 30, and 45 min after ACTH, but the sexes did not differ in response to the other stimuli. For individual rats, the maximum response to ACTH was slightly correlated with the maximum response to novelty stimulation, nicotine, and saline but was not correlated with the response to LPS.

Hormones, lymphohemopoietic cytokines and the neuroimmune axis

The classical distinction between hormones and cytokines has become increasingly obscure with the realization that homeostatic responses to infection involve coordinated changes in both the neuroendocrine and immune systems. The hypothesis that these systems communicate with one another is supported by the ever-accruing demonstrations of a shared molecular network of ligands and receptors. For instance, leukocytes express receptors for hormones and these receptors modulate diverse biological activities such as the growth, differentiation and effector functions. Leukocyte lineages also synthesize and secrete hormones, such as insulin-like growth factor-I (IGF-I), in response to both growth hormone (GH) and also to cytokines such as tumor necrosis factor-alpha (TNF-alpha). Since hormones share intracellular signaling substrates and biological activities with classical lymphohemopoietic cytokines, neuroendocrine and immune tissues share a common molecular language. The physiological significance of this shared molecular framework is that these homeostatic systems can intercommunicate. One important example of this interaction is the mechanism by which bacterial lipopolysaccharide, by eliciting a pro-inflammatory cytokine cascade from activated leukocytes, modulate pituitary GH secretion as well as other CNS-controlled behavioral and metabolic events. This article reviews the cellular and molecular basis for this communication system and proposes novel mechanisms by which neuroendocrine-immune interactions converge to modulate disease resistance, metabolism and growth.

Acute modulation of active carrier-mediated brain-to-blood transport of corticotropin-releasing hormone

The unidirectional brain-to-blood transport system for corticotropin-releasing hormone (CRH) across the blood-brain barrier could be instrumental in the homeostasis of central CRH. To characterize this system, the intracerebroventricular injection of 125I-CRH was used in mice. CRH was rapidly transported out of the brain with a half-time disappearance (t1/2) of 15 min, much faster than albumin (t1/2 = 50 min). Kinetic analysis revealed a saturable component with a low maximum velocity (apaproximately 0.020 nmol x min(-1) x brain(-1)) and low capacity (Michaelis constant approximately 1.4 nmol/brain). Transport was inhibited by verapamil, ouabain, and colchicine but not by cyclosporin. Transport was increased by corticosterone and inhibited by tumor necrosis factor-alpha and beta-endorphin. These results suggest that the specific unidirectional brain-to-blood transport system for CRH is dependent on energy and calcium channels, involves microtubules, is independent of the P-glycoprotein transporter, and is acutely modulated by adrenal steroids, cytokines, and endogenous opiates. This suggests its participation in the control of the stress response.

CRH and the immune system

Inflammatory cytokines released during immune system activation can stimulate the hypothalamic-pituitary-adrenal axis and cause increased secretion of corticotropin-releasing hormone (CRH), adrenocorticotropin and glucocorticoids. Identification of CRH peptide and mRNA, as well as its receptors in immune tissues, suggested a role for this peptide as a mediator of the neuroendocrine-immune interactions. Experimental evidence suggests that CRH may modulate the immune and inflammatory responses via two pathways: an antiinflammatory one operated by centrally released CRH, most likely through stimulation of glucocorticoid and catecholamine release, and one proinflammatory, through direct action of peripherally released CRH. This review highlights these concepts. In addition preliminary data on immune activation and inflammatory response in CRH-deficient mice created in our laboratory are discussed.

Systemic capsaicin pretreatment fails to block the decrease in food-motivated behavior induced by lipopolysaccharide and interleukin-1beta

The physiological and behavioral disturbances observed during an infection can be reproduced by systemic administration of proinflammatory cytokines (e.g., interleukin (IL)-1, IL-6, tumor necrosis factor-alpha) or lipopolysaccharide (LPS), a potent inducer of these cytokines. It is now well established that these molecules induce their effects by acting centrally, however, the mechanisms by which they reach central structures are not clear. We have earlier proposed that the humoral immune message is converted to a central neural activation by the action of cytokines on peripheral terminations of afferent neurons. Subdiaphragmatic vagotomy abolishes several effects of peripherally injected IL-1beta and LPS (e.g., decreased food-motivated behavior and social exploration, central expression of cytokines). To further define the nature of the peripheral fibers implicated in this phenomenon, we used a potent sensory neurotoxin, capsaicin, to selectively destroy C-fiber afferents. Adult rats were injected I.P. with a total dose of 25 mg/kg capsaicin in a series of 10 injections over a 48-h period. Adult mice were injected I.P. with a total dose of 75 mg/kg in a series of seven injections over a 7-day period. Although capsaicin treatment altered visceral chemosensory function, corneal and pain sensitivity, vagal-mediated anorexic effects of cholecystokinin, and depleted levels of substance P in the thoracic spinal cord, it was completely ineffective in blocking the decrease in food-motivated behavior induced by IL-1beta (4 microg/rat I.P. in rats) and LPS (250 microg/kg I.P. in rats and 400 microg/kg I.P. in mice). Thus, other afferents besides capsaicin-sensitive C-fibers appear to be involved in the transduction of cytokine effects during inflammatory and infectious events.

Bacterial endotoxin-induced gene expression in the choroid plexus and paraventricular and supraoptic hypothalamic nuclei of the sheep

The febrile and neuroendocrine responses to circulating endotoxin are effected, at least in part, by a central action of prostaglandins with interleukins serving as intermediaries. Data from rodents suggest that prostaglandin and interleukin (IL-1 beta) synthesis in response to endotoxin challenge may occur within the circumventricular organs of the brain, especially the choroid plexus; the present study investigated this possibility using the sheep as an experimental model. A pyretic dose of bacterial endotoxin (40 micrograms lipopolysaccharide) was given intravenously to sheep (n = 5) and the effect on gene expression in the choroid plexus after a 40 min interval was compared with that observed in vehicle-treated animals (n = 5) using in situ hybridisation histochemistry. Evidence of activational and synthetic events following endotoxin administration was provided by significant increases in c-fos (P < 0.05) and IL-1 beta (P < 0.01) mRNA expression. Constitutive cyclooxygenase (cox-1 mRNA) and inducible cyclooxygenase (cox-2 mRNA) synthesis were unchanged. The investigation also sought to provide evidence for endotoxin effects on neuroendocrine activity in this species by examining changes in hypothalamic gene expression. The results showed that c-fos mRNA increased in the paraventricular (P < 0.01) and supraoptic (P < 0.05) nuclei and that CRH mRNA was upregulated in the paraventricular nucleus (P < 0.001). However, in agreement with previous work, there was no change in vasopressin gene expression although oxytocin mRNA was enhanced throughout the paraventricular nucleus (P < 0.05). These findings suggest the following: (1) possible involvement of the choroid plexus in the response of sheep to immunological challenge: (2) endotoxin-induced changes in gene expression in the ovine hypothalamus similar in those caused by other stressors: and (3) possible changes in oxytocin synthesis concomitant with fever in the sheep.

Molecular characterization of immune derived proenkephalin mRNA and the involvement of the adrenergic system in its expression in rat lymphoid cells

Proenkephalin (PENK), a classically defined opioid gene, was originally thought to be expressed almost exclusively in the mature nervous and neuroendocrine systems. In the last few years, it was demonstrated, however, that significant levels of PENK mRNA and PENK-derived peptides are transiently expressed in cells of the immune system. Very little is known about the molecular mechanisms regulating this transient expression. In order to investigate those mechanisms, we examined the in vivo expression of PENK mRNA in mesenteric lymph nodes after exposing rats to lipopolysaccharide. In the present study we demonstrate that: (i) promoter usage and splicing of PENK mRNA function similarly in mesenteric lymph nodes as in neural cells; (2) PENK expression in mesenteric lymph nodes is modulated by adrenaline via adrenergic receptors; and (3) the adrenergic system participates in the modulation of the LPS induced PENK mRNA expression. These results provide more evidence for the involvement of opioids in neuro-immune interactions.

Subdiaphragmatic vagotomy inhibits intra-abdominal interleukin-1 beta stimulation of adrenocorticotropin secretion

Although interleukin (IL)-1 beta activates the hypothalamic-pituitary-adrenal (HPA) axis, the mechanisms by which peripheral IL-1 beta acutely stimulates adrenocorticotropin (ACTH) secretion are not clear. Recently, the vagus has been implicated in mediating peripheral cytokine signalling of the brain. To investigate a possible central mechanism for peripheral cytokine stimulation of the HPA axis, we tested the hypothesis that the vagus mediates IL-1 beta activation of the HPA axis by an intra-abdominal stimulus. We studied the effect of subdiaphragmatic vagotomy on plasma ACTH stimulation in rats by intraperitoneal (i.p.) IL-1 beta. Adult male Sprague-Dawley rats underwent subdiaphragmatic vagotomy or sham surgery 1 week prior to study. Rats were killed 1 and 2 h after i.p. saline (control) and low- (4 micrograms/kg) and high-dose (20 micrograms/kg) IL-1 beta. Vagotomy markedly attenuated plasma ACTH secretion at 2 h after high-dose IL-1 beta stimulation and abolished plasma ACTH secretion at 2 h after low-dose IL-1 beta stimulation. At 1 h after low-dose IL-1 beta, stimulation of plasma ACTH in vagotomized animals was also markedly diminished compared to sham animals. However, vagotomy did not alter stimulation of plasma corticosterone at 1 or 2 h after low-dose IL-1 beta or at 2 h after high-dose IL-1 beta. In addition, vagotomy did not alter stimulation of plasma ACTH or corticosterone secretion by insulin-induced hypoglycemia. We conclude that: (1) the vagus plays an important role in stimulation of ACTH secretion by intra-abdominal (i.p.) IL-1 beta; (2) stimulation of corticosterone secretion by i.p. IL-1 beta is not altered by vagotomy; and (3) the inhibitory effect of vagotomy on activation of the HPA axis appears to be specific for immune stimulation by cytokines.

Peripheral administration of lipopolysaccharide induces activation of microglial cells in rat brain

Using immunocytochemistry with monoclonal antibodies against surface immunomolecules and Griffonia simplicifolia lectin histochemistry, the microglial cell reaction in rat brain was studied after intravenous injection of lipopolysaccharide (LPS). Activation of microglial cells throughout the brain became apparent within hours and peaked at 8-24 h following administration of 1, 2.5 and 5 mg/kg LPS. High doses of LPS (2.5 and 5 mg/kg) induced a morphological transition of resting ramified microglia to round, macrophage-like cells in the anterior hypothalamus, thalamus and the brainstem. After injection of 1 mg/kg LPS, this morphological transition was only detected in the brainstem. Microglial cell reactivity gradually returned to control levels within 7 days after LPS administration. Furthermore, LPS induced enhanced expression of MHC class II by microglial cells. Maximal up-regulation of MHC class II Ia-antigen was found 3 days following injection of LPS, and only a few highly Ia immunoreactive cells were detectable 7 days following injection of LPS. Despite the presence of highly activated microglial cells in the rat brain, no signs of tissue damage were observed at any time point after injection of LPS examined. In addition to the activation of microglial cells, intravenous injection of LPS induced accumulations of macrophages in blood vessels of the choroid plexus and the brain, but no disruption of vessels with subsequent invasion of parenchyma by blood macrophages was detected. Our data demonstrate that a peripheral immune challenge leads to a high and transitory activation of microglial cells in the brain which could possibly contribute to the pathology of infections and septic shock.

The protective role of the hypothalamic-pituitary-adrenal axis against lethality produced by immune, infectious, and inflammatory stress

We have shown that ADX and HYPOX rats exhibit a markedly increased sensitivity to the lethal effects of IL-1-beta and LPS compared to sham controls with an intact HPAA. These results indicated that the reports of lethal effects of cytokines and LPS which generates cytokines in mice with a compromised HPAA were not idiosyncratic or specific to mice but represented a general response that would have been expected in any organism with a compromised HPAA. We further demonstrated that protection against lethal effects due to IL-1-beta or LPS could be produced by treating ADX rats with glucocorticoid in a quantity estimated to be equivalent to corticosterone secretion provoked during stress. In contrast, we found that acutely stalk-sectioned rats with pituitaries disconnected from hypothalamic regulation did not show a markedly increased susceptibility to lethal effects of LPS as did ADX or HYPOX rats. Although a minority of stalk-sectioned rats were killed by LPS, the majority of rats were protected from lethal actions of LPS. This response suggested that an intact pituitary-adrenal axis without the normal hypothalamic control could still provide significant protection presumably due to generation of cytokines which stimulated the pituitary over several hours. The results from our lethality studies clearly underscore the importance of activating the stress axis and increasing glucocorticoid secretion to protect against potentially lethal effects of cytokines that can be induced by immune, infectious, or inflammatory stimuli. Cytokine-stimulated effects can initially result in beneficial actions to the host by promoting immune/inflammatory responses that are protective in nature and help defend against a variety of invading stimuli (infectious, immune, inflammatory, traumatic, neoplastic). Normally the HPAA responds to cytokine stimulation by ultimately increasing glucocorticoid secretion in order to counterregulate cytokine actions, modulate the host response, and protect the host from excessively catabolic effects of unregulated cytokine generation and actions. For many years, clinicians have recognized that patients with deficient glucocorticoid secretion (e.g., Addison's disease or pituitary ACTH deficiency) require increased glucocorticoid replacement during episodes of fever, infection, or inflammatory stress. However, the reasons why stress-equivalent glucocorticoid replacement were required were not entirely clear. Now, we understand that glucocorticoids are critically important for protecting the host against its own defense mechanisms so that the stimulation of cytokines can facilitate a protective response against an invading insult without also killing the host.

Systemic endotoxin increases steady-state gene expression of hypothalamic nitric oxide synthase: comparison with corticotropin-releasing factor and vasopressin gene transcripts

The enzyme responsible for nitric oxide (NO) formation, NO synthase (NOS), is found in hypothalamic neurons that control ACTH secretion. This led to the hypothesis that brain NO may modulate the response of the hypothalamic-pituitary (HP) axis to various stimuli. We tested this hypothesis by measuring changes in constitutive (c) NOS mRNA levels in the hypothalamus of rats systemically injected with endotoxin, a lipopolysaccharide (LPS) that releases endogenous cytokines, and analyzed these results in the context of the appearance of ACTH-releasing secretagogues such as corticotropin-releasing factor (CRF) and vasopressin (VP), as well as CRF receptors type A (CRF-RA). We purposefully chose doses of LPS thought to only minimally disrupt the blood-brain barrier and not be accompanied by an endotoxin shock, so that the results we obtained did not primarily stem from abnormal passage of compounds into the brain, or non-specific stress. Three to four hours following LPS injection (100 micrograms/kg, i.v.), cNOS mRNA levels increased in the paraventricular nucleus (PVN) of the hypothalamus. LPS treatment also upregulated PVN CRF gene transcription (measured by CRF heteronuclear RNA) and increased steady-state gene expression of the immediate early genes (IEG) c-fos and NGFI-B, with the first changes noted 1-2 h after treatment. Transcripts of CRF receptors type A were present in the hypothalamus 6 h after endotoxin treatment. On the other hand, no alterations in cytoplasmic VP mRNA levels were noted in rats injected with LPS. Because the dose of LPS we used stimulates ACTH secretion within 30 min, our results suggest that systemic LPS acts first within the median eminence, where it stimulates peptidic nerve terminals. Neuronal activation of hypothalamic cell bodies takes place later, and whether this phenomenon is due to the production of brain neurotransmitters and/or cytokines, or whether it primarily results from increased demand on the synthetic machinery, remains to be established. These studies extend prior work showing that systemic LPS increases the neuronal activity of hypothalamic regions known for their involvement in the responses of the HP axis, and bring forth two important additional points. First, increases in CRF primary nuclear transcripts are delayed with regard to the temporal release of ACTH. This suggests, though it does not demonstrate, that under the experimental conditions we used, the first site of action of LPS is the median eminence. Second, the observation of increased cNOS gene expression following LPS treatment, and the presence of this enzyme in neurons that regulate ACTH secretion, bring support to the hypothesis that this gas plays an important function in mediating the HP axis response to an immune challenge.

Effects of HSV-1, a neurotropic virus, on the hypothalamic-pituitary-adrenocortical axis in rats

It is well established that the hypothalamic-pituitary-adrenocortical (HPA) axis is activated during systemic viral diseases. In this study we examined the effects of a neurotropic virus, herpes simplex virus type 1 (HSV-1), on the HPA axis in male rats. Following corneal inoculation with HSV-1, the virus invaded the nervous system and replicated in the brainstem without clinical signs of disease. During this asymptomatic brainstem infection with HSV-1, significant changes were found in the function of the HPA axis: On days 3, 7 and 14 post-infection (p.i.) basal ACTH and corticosterone (CS) levels were markedly elevated, and photic stressful stimulation failed to further increase the levels of these hormones. In addition, the elevated basal serum levels of ACTH and CS could not be suppressed by pretreatment with dexamethasone. The content of CRF-41 in the paraventricular nucleus of the hypothalamus and in the median eminence measured at 6 days p.i. was similar to that of vehicle inoculated rats. By 4 weeks p.i. the basal levels of ACTH and CS returned to normal and these animals responded to photic stimulation and dexamethasone similar to vehicle inoculated rats. Systemic (intraperitoneal) inoculation of HSV-1 did not induce any changes in the HPA axis responses. We therefore suggest that asymptomatic acute infection of the brainstem with HSV-1 may affect brain regions involved in the regulation of the HPA axis, and that those effects are mediated centrally and not by a systemic mechanism.

Adult male rats exposed to an alcohol diet exhibit a blunted adrenocorticotropic hormone response to immune or physical stress: possible role of nitric oxide

Intact adult male rats fed an alcohol [ethanol (EtOH)] diet for 10 days show blunted adrenocorticotropic hormone (ACTH) release in response to immune signals such as the cytokine interleukin-1 beta (IL-1 beta) and endotoxin [lipopolysaccharide (LPS)], as well as to physical stress (mild electroshocks). The mechanisms responsible for this effect remain poorly understood, but we have recently reported that decreased pituitary responsiveness to vasopressin (VP) might play a role. In naive rats, nitric oxide (NO) exerts a restraining influence on the response of the hypothalamic-pituitary (H-P) axis to cytokines and VP. The ability of long-term EtOH treatment to increase glutamate receptors, and thus NO formation, prompted us to test the hypothesis that abnormally high NO concentrations might modulate the influence of the drug. Blockade of the activity of NO synthase (NOS), the enzyme responsible for NO formation, with the arginine derivative L-N omega nitro-L-arginine-methylester (L-NAME), augmented the ACTH response to IL-1 beta or LPS in both control (C) and EtOH-fed (E) rats. Indeed, after L-NAME treatment, ACTH concentrations were statistically comparable in C and E animals injected with endotoxin or a large dose of IL-1 beta. VP-induced ACTH secretion also became comparable in both experimental groups after blockade of NOS activity. In contrast, the decreased response of the H-P axis of E animals to shocks was only slightly ameliorated, compared with that of C rats. It is therefore possible that changes in the NOergic tone induced by alcohol play a role in the decreased response of the H-P axis to cytokines, possibly in part by altering the stimulatory action of VP on the corticotrophs. On the other hand, in E rats NO seems to exert only a minimal influence on the central nervous system circuits activated by shocks.

Intraperitoneal administration of bacterial endotoxin enhances noradrenergic neurotransmission in the rat preoptic area: relationship with body temperature and hypothalamic--pituitary--adrenocortical axis activity

A combined in vivo microdialysis/biotelemetry method in freely moving rats was used to study the effects of an endotoxic challenge on brain neurotransmission, hypothalamic-pituitary-adrenocortical (HPA) axis activity, autonomic functions and behaviour. Rats were equipped with a microdialysis probe in the preoptic area and a transmitter for biotelemetry in the peritoneal cavity. Time-dependent changes in noradrenergic and serotonergic neurotransmission, and HPA axis activity were monitored by measuring noradrenaline, serotonin, their metabolites and free corticosterone concentrations in dialysates. Core body temperature, heart rate and locomotion were measured simultaneously by biotelemetry. In addition, total behavioural activity was scored by measuring the time during which rats were active. Intraperitoneal administration of endotoxin (lipopolysaccharide; 100 micrograms/kg body weight) caused a pronounced increase in preoptic extracellular concentrations of noradrenaline and its metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG; 500 and 400% of baseline respectively). No effect was found on preoptic concentrations of serotonin, although the levels of its metabolite 5-hydroxyindoleacetic acid were slightly elevated (120% of baseline). Intraperitoneal lipopolysaccharide caused a marked increase in corticosterone levels, a decline in behavioural activity, and biphasic rises in body temperature and heart rate. Analysis of the time curves revealed that noradrenaline rose in parallel with the first increase in body temperature and the increase in corticosterone levels. Moreover, maximum noradrenaline levels were reached approximately 60 min earlier than the peak in body temperature and corticosterone concentrations. Intraperitoneal pretreatment with the cyclo-oxygenase inhibitor indomethacin prevented the lipopolysaccharide-induced changes in body temperature, heart rate and behavioural activity, whereas the changes in noradrenaline, MHPG and corticosterone were largely, but not completely, reduced. Taken together, the results show that an endotoxic challenge results in a highly differentiated response in brain neurotransmission. We postulate that the profound increase in preoptic noradrenergic neurotransmission may be related to the lipopolysaccharide-evoked induction of fever and/or activation of the HPA axis.

Deletion of the ventral noradrenergic bundle obliterates the early ACTH response after systemic LPS, independently from the plasma IL-1β surge

We have recently shown that total lesion of the ventral noradrenergic bundle (VNAB-X), enhanced the short-lived (<120 min) triggering effect of intra-arterially (i.a.) given IL-1β on plasma ACTH levels. In the present study we used the same VNAB-X paradigm to explore the mechanisms of the long-lived (480 min) LPS stimulatory effect on plasma ACTH, corticosterone (CORT) and IL-1β levels. In control rats, 25 μg kg(-1) LPS induced a 20-fold increase in ACTH and a 7-fold increase in CORT concentrations at 30 min, which continued to rise until 60 min, before receding to baseline at 480 min. In contrast, the plasma IL-1β concentration started to increase above undetectable levels only at 120 min. In VNAB-X animals, the early (30 min) ACTH/CORT response to LPS was completely blunted, and the ACTH surge was reduced by 75% at 60 min. However, the sustained hormonal response (120 to 480 min) was unaltered. Both the temporal pattern and the amplitude of the plasma IL-1β response were normal. We conclude that (1) the VNAB is involved in the early (first 60 min) ACTH/CORT response to systemic LPS, (2) plasma IL-1β does not appear to be associated with this early corticotropic activation and (3) the later stages of the ACTH/CORT response to LPS (60 to 480 min) appear to be independent of the VNAB control and may therefore involve different control mechanisms, in which the IL-1β, by this stage massively released in the blood, may play a major role.

Bacterial lipopolysaccharide (LPS) and interleukin 1 (IL-1) exert multiple physiological effects in the tilapia Oreochromis mossambicus (Teleostei)

To gain insight in immuno-endocrine communication in teleosts the physiological effects of interleukin 1 and bacterial lipopolysaccharide in teleosts were investigated. Tilapia (Oreochromis mossambicus) were treated with murine interleukin 1 and E. coli lipopolysaccharide in vivo, and lipopolysaccharide was administered to pituitary lobes and head kidneys in vitro. The integument of the fish appeared to be a sensitive target for the preparations tested, since proliferation of chloride cells and of epidermal mucous cells was observed as well as an increase in epidermal thickness. These effects may relate to an acute phase-like reaction caused by the treatments. Lipopolysaccharide administration furthermore resulted in an increase in plasma free fatty acids levels. Lipopolysaccharide, but not interleukin 1, stimulated the interrenal axis of the fish, as judged by the increase in cortisol production measured in superfusion of head kidneys. In addition to these in vivo effects, lipopolysaccharide also displayed several effects in vitro. Pituitary adrenocorticotropic hormone, as well as alpha-melanocyte stimulating hormone, release was inhibited, and the head kidney responsiveness to adrenocorticotropic hormone was inhibited after pretreatment of the tissue with the E. coli product. This latter effect coincided with the release of an unidentified alpha-melanocyte stimulating hormone immunoreactive fraction by the head kidneys which could be stimulated by lipopolysaccharide. The data strongly support the notion that the immune system is involved in adaptive regulations in teleosts, and that immunoendocrine interactions are phylogenetically old mechanisms.

Effects of monoclonal antibodies to specific epitopes of rat interleukin-1 beta (IL-1 beta) on IL-1 beta-induced ACTH, corticosterone and IL-6 responses in rats

Recently, we developed a panel of monoclonal antibodies (MoAbs) to rat IL-1 beta and found that MoAbs binding to the aminoacid sequences 66-85 and 123-143 of mature rIL-1 beta inhibited the binding of rIL-1 beta to murine EL4 cells. Here we study whether MoAbs to these and other domains of IL-1 interfere with the biological effects of rIL-1 beta in adult male rats in vivo. Administration of rIL-1 beta (1 or 5 micrograms/kg i.v.) enhanced the plasma concentrations of ACTH, corticosterone (CORT) and of IL-6 in a time- (0.5-4 h) and dose-dependent manner. Because 2 h after 5 micrograms/kg i.v., all three parameters were consistently elevated, this dose and time interval was used for further studies. Prior to injection, rIL-1 beta was incubated alone or in the presence of a MoAb (10 mg/kg) for 30 min at 37 degrees C or at 4 degrees C. Plasma ACTH, CORT and IL-6 responses to these mixtures are compared to those obtained after preincubation of rIL-1 beta with a non-IL-1 binding MoAb (PEN7). SILK 3, a MoAb that binds to the 66-85 domain of rIL-1 beta, reduced the ACTH and IL-6 responses by 48 and 45% respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Endotoxin-induced appearance of immunoreactive interleukin-1 beta in ramified microglia in rat brain: a light and electron microscopic study

Interleukin-1 plays an important role as mediator of endotoxin-induced responses in the brain such as fever, sleep, anorexia, behavioural and neuroendocrine changes. In the present study, interleukin-1 beta immunocytochemistry has been performed at the light and electron microscopic level to study the cellular and subcellular localization of interleukin-1 beta in the brains of rats given endotoxin or saline. Light microscopic analysis of rats killed 4, 8 or 24h after endotoxin (2.5 mg/kg) given intraperitoneally or intravenously revealed a region-specific localization of immunoreactive interleukin-1 beta in macrophages and microglial cells. After saline treatment, no induction of interleukin-1 beta immunoreactivity occurred in the brain. After administration of endotoxin, many interleukin-1 beta-positive cells were found in the meninges, choroid plexus, circumventricular organs, cerebral cortex and hypothalamus. The number of interleukin-1 beta-positive microglial cells reached a maximum 8 h after administration of endotoxin, irrespective of the route of administration. In general, more interleukin-1 beta-positive microglial cells were found after intravenous than after intraperitoneal administration of endotoxin. Interleukin-1 beta-positive microglial cells were often grouped in patches in the vicinity of blood vessels. At the surface of the cerebral cortex, in the meninges, intermediate cell forms between interleukin-1 beta-positive macrophages and microglial cells were found. interleukin-1 beta-positive perivascular microglia were localized at the brain side of the basal lamina. Immunoreactive interleukin-1 beta was found at the luminal side of the endothelial cells lining the venules. Furthermore, microglial cells that extended their processes into the ependymal layer of the third ventricle were observed. Results of the electron microscopic studies revealed immunoreactive interleukin-1 beta in many cells with the cellular characteristics of microglial cells, but also, in some cells, identified as astrocytes. In microglial cells, immunoreactive interleukin-1 beta was found in the cytoplasm but not in the endoplasmatic reticulum or Golgi apparatus. These results show that after peripheral administration of endotoxin, immunoreactive interleukin-1 beta is induced in macrophages in the meninges and in the choroid plexus, as well as in microglial cells in parenchyma. Interleukin-1 beta produced by these cells may serve as a signal for adjacent or more distant targets (neurons, endothelial cells, microglial cells) to play a role in the induction of non-specific symptoms of sickness.

Pituitary-adrenal and interleukin-6 responses to recombinant interleukin-1 in neonatal rats

The cytokine interleukin-1 (IL-1) is a potent activator of the hypothalamic-pituitary-adrenal (HPA) axis. During postnatal development, the rat appears to be hyporesponsive to many stimuli which activate the HPA system in adulthood. Since hyporesponsiveness depends to a large extent on the stimulus, these experiments investigated the ontogeny of the HPA axis and interleukin-6 (IL-6) responses to IL-1 beta. Six-, 9-, and 18-day-old pups were injected with human recombinant IL-1 beta and plasma ACTH, corticosterone (CORT) and IL-6 levels were measured. IL-1 beta administration resulted in age-dependent endocrine and immune responses. The younger neonates secreted less ACTH and CORT and more IL-6. This was not due to a lowered capacity of the pituitary to synthesize and secrete ACTH since peptide levels following adrenalectomy did not reveal age differences. These data suggest that the diminished response to IL-1 beta is due to the immaturity of neural circuits which may be required to fully activate the HPA axis to immune signals.