ACTH response induced in capsaicin-desensitized rats by intravenous injection of interleukin-1 or prostaglandin E

Antioxidants increase the ventilatory response to hyperoxic hypercapnia

RATIONALE

The mechanisms by which chemoreceptors process carbon dioxide stimuli are poorly understood. Recent in vitro studies suggest a role of reactive oxygen species in central carbon dioxide chemoreception.

OBJECTIVES

We tested the hypothesis that antioxidant treatment modulates the ventilatory response to carbon dioxide in healthy humans, either during unloaded breathing or after strenuous resistive breathing.

METHODS

In the first experiment of this randomized, double-blind, placebo-controlled study, 14 healthy males completed hyperoxic carbon dioxide rebreathing, received either antioxidants (vitamins E, A, and C for 2 mo, allopurinol for 15 d, and N-acetylcysteine for 3 d) (n = 7) or placebo (n = 7), and repeated rebreathing 3 mo later. In the second experiment, 18 healthy males completed a series of rebreathing tests before and after strenuous resistive breathing. Subjects repeated the same protocol 3 mo later, after they had received antioxidants (n = 9) or placebo (n = 9).

MAIN RESULTS

After antioxidants, the sensitivity of the ventilatory (minute ventilation) response to carbon dioxide increased (mean [+/- SEM], 3.2 +/- 0.5 vs. 1.7 +/- 0.4 L/min/mm Hg; p < 0.001). Antioxidants also increased the sensitivity to carbon dioxide before and at 5, 30, and 120 min after resistive breathing (p = 0.01). This effect was entirely due to increased tidal volume. Antioxidants did not influence the breathing pattern during resting breathing or the rapid shallow breathing response to carbon dioxide at 5 min after resistive breathing.

CONCLUSIONS

Antioxidants, by augmenting the tidal volume, increase the sensitivity of the ventilatory response to carbon dioxide, either during unloaded breathing or after resistive breathing.

Is loaded breathing an inflammatory stimulus?

PURPOSE OF REVIEW

To summarize recent data indicating that loaded breathing generates an inflammatory response.

RECENT FINDINGS

Loaded breathing initiates an inflammatory response consisting of elevation of plasma cytokines and recruitment and activation of lymphocyte subpopulations. These cytokines do not originate from monocytes but are instead produced within the diaphragm secondary to the increased muscle activation. Oxidative stress is a major stimulus for the cytokine induction secondary to loaded breathing. The production of cytokines within the diaphragm may mediate the diaphragm muscle fiber injury that occurs with strenuous contractions, or contribute to the expected repair process. These cytokines may also compromise diaphragmatic contractility or contribute to the development of muscle cachexia. They may also have systemic effects, mobilizing glucose from the liver and free fatty acids from the adipose tissue to the strenuously working respiratory muscles. At the same time, they stimulate the hypothalamic-pituitary-adrenal axis, leading to the production of adrenocorticotropic hormone and beta-endorphins. The adrenocorticotropic hormone response may represent an attempt of the organism to reduce the injury occurring in the respiratory muscles through the production of glucocorticoids and the induction of the acute-phase response proteins. The beta-endorphin response would decrease the activation of the respiratory muscles and change the pattern of breathing, which becomes more rapid and shallow, possibly in an attempt to reduce and/or prevent further injury to the respiratory muscles.

SUMMARY

Loaded breathing is an immune challenge for the body, initiating an inflammatory response. Further studies are needed to elucidate the role of this response in the development of ventilatory failure.

Intracerebral HIV-1 glycoprotein 120 produces sickness behavior and pituitary-adrenal activation in rats: role of prostaglandins

HIV infection is associated with profound neurobehavioral and neuroendocrine impairments. Previous studies demonstrated that HIV causes neuropathological alterations indirectly, via shedding of glycoprotein 120 (gp120) within the brain. To extend these findings, we examined the neurobehavioral and neuroendocrine effects of central administration of gp120, as well as the role of brain prostaglandins in mediating these effects. Intracerebroventricular (i.c.v.) injection of gp120 in rats produced a marked sickness behavior syndrome, consisting of reduced exploratory behavior, suppressed consumption of food and saccharin solution, and reduced body weight. Gp120 also induced a significant febrile response and increased serum levels of ACTH and corticosterone. Following i.c.v. gp120 administration, the ex vivo production of PGE2 by the hypothalamus, frontal cortex, and hippocampus was significantly elevated, and indomethacin, a prostaglandin synthesis inhibitor, attenuated this elevation. Pre-treatment with indomethacin reduced the fever and adrenocortical activation induced by gp120 administration, but not its behavioral effects. These findings indicate that gp120 may be responsible for some of the behavioral and endocrine abnormalities seen in HIV-infected patients. Prostaglandins are important mediators of the physiological, but not the behavioral effects of brain gp120.

Fever induction pathways: evidence from responses to systemic or local cytokine formation

The immune and central nervous systems are functionally connected and interacting. The concept that the immune signaling to the brain which induces fever during infection and inflammation is mediated by circulating cytokines has been traditionally accepted. Administration of bacterial lipopolysaccharide (LPS) induces the appearance of a so-termed "cytokine cascade" in the circulation more or less concomitantly to the developing febrile response. Also, LPS-like fever can be induced by systemic administration of key cytokines (IL-1 beta, TNF-alpha, and others). However, anti-cytokine strategies against IL-1 beta or TNF-alpha along with systemic injections of LPS frequently lead to attenuation of the later stages of the febrile response but not of the initial phase of fever, indicating that cytokines are rather involved in the maintenance than in the early induction of fever. Within the last years experimental evidence has accumulated indicating the existence of neural transport pathways of immune signals to the brain. Because subdiaphragmatic vagotomy prevents or attenuates fever in response to intraperitoneal or intravenous injections of LPS, a role for vagal afferent nerve fibers in fever induction has been proposed. Also other sensory nerves may participate in the manifestation of febrile responses under certain experimental conditions. Thus, injection of a small dose of LPS into an artificial subcutaneous chamber results in fever and formation of cytokines within the inflamed tissue around the site of injection. This febrile response can be blocked in part by injection of a local anesthetic into the subcutaneous chamber, indicating a participation of cutaneous afferent nerve signals in the manifestation of fever in this model. In conclusion, humoral signals and an inflammatory stimulation of afferent sensory nerves can participate in the generation and maintenance of a febrile response.

Afferent nerves are involved in the febrile response to injection of LPS into artificial subcutaneous chambers in guinea pigs

In guinea pigs, fever was induced by injections of 100 or 10 microgram/kg lipopolysaccharide (LPS) into artificial subcutaneous chambers and analysed under the influence of the local anesthetic, ropivacaine (ROPI), which was administered into the chamber at a dose of 10 mg/kg 30 min prior to LPS. In response to injections of 100 microgram/kg LPS into the subcutaneous chambers, fever was not modified by pretreatment with ROPI. High amounts of bioactive tumor necrosis factor (TNF) alpha and interleukin-6 (IL-6) were measured in the lavage of the chambers after administration of LPS. Comparatively low concentrations of both cytokines (0.5-4% of the concentrations in the lavage fluid) were detected in blood plasma simultaneously. In response to injections of 10 microgram/kg LPS into the subcutaneous chambers, fever was significantly reduced by pretreatment with ROPI to about 60% of the febrile response of control animals. Levels of TNF and IL-6 were lower in response to the reduced dose of LPS. TNF in plasma was even below the limit of detection. The suppression of fever by the local anesthetic was not observed when ROPI was subcutaneously injected into the contralateral site of the chamber position so that a systemic effect of ROPI in the reduction of fever can be excluded. The results indicate a participation of afferent neural signals in the manifestation of fever. This effect becomes obvious only if the dose of the applied inflammatory stimulus (LPS) is not high enough to activate a systemic generalised inflammatory response.

Vagotomy does not affect thermal responsiveness to intrabrain prostaglandin E2 and cholecystokinin octapeptide

Subdiaphragmatic vagotomy has been repeatedly shown to attenuate the febrile response to peripherally injected pyrogens. In the present study, we investigated whether vagotomy-induced attenuation of febrile responsiveness reflects a decreased sensitivity of the brain to central fever mediators, prostaglandin E2 (PGE2) and cholecystokinin octapeptide (CCK-8). Male rats were subjected to subdiaphragmatic vagotomy (or sham surgery) on day 0 and had a cannula implanted into the lateral cerebral ventricle on day 24. On day 30-36, the thermal responsiveness of the rats to PGE2 or CCK-8 was tested. Each animal was injected in the ventricle with either PGE2 (0, 10, 100, or 500 ng) in pyrogen-free saline with 0.5% ethanol (5 microl) or CCK-8 (0 or 1.6 microg) in artificial cerebro-spinal fluid (5 microl). While the 0-dose of either PGE2 or CCK-8 (vehicle alone) induced no thermal response, all the higher doses of either agent caused a body temperature rise preceded by tail skin vasoconstriction. The vagotomized rats did not respond differently than their sham-operated counterparts to any dose of either drug. It is concluded that subdiaphragmatic vagotomy does not change the rat's thermal responsiveness to intrabrain PGE(2) and CCK-8.

Strenuous resistive breathing induces proinflammatory cytokines and stimulates the HPA axis in humans

Interleukin-1beta (IL-1beta) and interleukin-6 (IL-6), powerful stimulants of the hypothalamic-pituitary-adrenal (HPA) axis, increase in response to whole body exercise. Strenuous inspiratory resistive breathing (IRB), a form of clinically relevant "exercise" for the respiratory muscles, produces beta-endorphin through a largely unknown mechanism. We investigated (in 11 healthy humans) whether strenuous IRB produces proinflammatory cytokines and beta-endorphin in parallel with stimulation of the HPA axis, assessed by concurrent measurement of ACTH. Subjects underwent either severe [at 75% of maximal inspiratory pressure (P(m) (max))] or moderate (at 35% of P(m) (max)) IRB. Plasma cytokines, beta-endorphin, and ACTH were measured at rest (point R), at the point at which the resistive load could not be sustained (point F), and at exhaustion [15 min later (point E)]. During severe IRB, IL-1beta increased from 0.83 +/- 0.12 pg/ml at point R to 1.88 +/- 0. 53 and 4.06 +/- 1.27 pg/ml at points F and E, respectively (P < 0. 01). IL-6 increased from 5.30 +/- 1.02 to 10.33 +/- 2.14 and 11.66 +/- 2.29 pg/ml at points F and E, respectively (P = 0.02). ACTH and beta-endorphin fluctuated from 20.87 +/- 5.49 and 25.03 +/- 3.97 pg/ml at point R to 22.97 +/- 4.41 and 26.32 +/- 3.93 pg/ml, respectively, at point F and increased to 46.96 +/- 8.55 and 40.32 +/- 5.94 pg/ml, respectively, at point E (P < 0.01, point E vs. point F). There was a positive correlation between the IL-6 at point F and the ACTH and beta-endorphin at point E (r = 0.88 and 0.94, respectively; P < 0.01) as well as between the increase in IL-6 (between points R and F) and the increases in ACTH and beta-endorphin (between points F and E, r = 0.91 and 0.92, respectively; P < 0.01). Moderate IRB did not produce any change. We conclude that severe IRB produces proinflammatory cytokines and stimulates the HPA axis in humans secondary to the production of cytokines (especially IL-6).

Evidence that cholecystokinin receptors are not involved in the hypothalamic-pituitary-adrenal response to intraperitoneal administration of interleukin-1beta

The purpose of this study was to investigate a putative role for cholecystokinin (CCK) in the activation of the hypothalamic-pituitary-adrenal (HPA) axis following intraperitoneal (i.p.) administration of interleukin-1-beta (IL-1beta). Previous studies predict that CCKA receptors on vagal sensory afferents may be involved in the initiation of the stress response following an acute i.p. injection of IL-1beta. Adult male rats were given an i.p. injection of a specific CCKA (devazepide, 1 mg/kg) or CCKB (CI-988, 1 mg/kg) receptor antagonist, 30 min prior to an i.p. injection of rat recombinant IL-1beta (rrIL-1beta), 0.5 microg/kg in 0.9% sterile saline/0.01% rat serum albumin. Blood samples were obtained via an indwelling jugular vein catheter, and the plasma levels of the stress hormones ACTH (adrenocorticotropin hormone) and corticosterone analysed over time as an indicator of HPA axis activation. This dose of rrIL-1beta resulted in a significant release of ACTH and corticosterone, peaking at 30-60 min, and returning to basal levels by 2 h. Pretreatment with either devazepide or CI-988 had no effect on the rrIL-1beta induced ACTH or corticosterone release. In contrast, the same dose of devazepide completely inhibited the ACTH and corticosterone response to i.p. CCK (octapeptide, sulphated form, CCK-8S), 5 microg/kg. It is concluded that CCK receptors are not involved in the hormonal stress response to a submaximal i.p. dose of rrIL-1beta.

Cytokines inhibit sexual behavior in female rats: II. Prostaglandins mediate the suppressive effects of interleukin-1beta

The proinflammatory cytokine interleukin-1 (IL-1) induces several behavioral alterations that are characteristic of illness, such as anorexia and reduced locomotor and social activity. We have recently demonstrated that IL-1 inhibits sexual activity, motivation and attractivity in female, but not in male rats following either central or peripheral administration. In the present study we examined the involvement of prostaglandin (PG) synthesis in mediating IL-1-induced suppression of female sexual behavior. Administration of the cyclooxygenase blockers indomethacin or ibuprofen completely prevented IL-1-induced suppression of female sexual behavior, including the reduction in proceptive behavior, the lordosis response to a male's mounts, and the preference for a sexually active partner. In a subsequent study, ex-vivo release of hypothalamic PGE2 and the secretion of corticosterone (CS) were measured in males and estrous females following IL-1 administration. At the same time and dose of IL-1 administration that significantly reduced sexual behavior in female but not male rats, IL-1 produced a significant increase in PGE2 release in female, but not in male rats. In contrast, IL-1 induced a significant elevation of serum CS levels in males but not in females. These findings suggest that PG synthesis is involved in mediating the effects of IL-1 on female sexual behavior. Furthermore, differential secretion of PGs and CS may underlie the gender difference in the effects of IL-1 on sexual behavior.

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.

Pyretic and antipyretic signals within and without fever: a possible interplay

Current concepts on the pathogenesis of fever emphasize the importance of the cytokine-prostaglandin cascade. This humoral line mediates nonthermal signals to the brain, while the thermal signals supply feedback from the thermoreceptors. However, the humoral line cannot alone account for the whole febrile response. Here, we hypothesize that, besides this humoral mediatory mechanism, nonthermal neural signals of abdominal origin conveyed mainly by the vagus nerve are also important pro-pyretic factors. The pro-pyretic mechanisms are proposed to be in a dynamic balance with endogenous antipyretic mechanisms that also form an integral part of the normal reaction to pyrogens. Further, it is hypothesized that the role of such neural and humoral signals either for elevation or depression of body temperature is not limited to fever but has an important role also in nonfebrile thermoregulation.

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

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

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.

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

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

Elevation of plasma ACTH concentration in rabbits made febrile by systemic injection of bacterial endotoxin

This study was carried out to investigate the adrenocorticotropic hormone (ACTH) response in rabbits made febrile by systemic injection of lipopolysaccharide (LPS, Salmonella typhosa endotoxin). Intravenous (i.v.) injection of LPS (0.1 microgram/kg and 1.0 microgram/kg) increased rectal temperature (biphasic fever) and the plasma concentration of ACTH (ACTH response) in a dose-related manner. These responses were suppressed by pretreatment with indomethacin (20 mg/kg, subcutaneously). Intracerebroventricular (i.c.v.) administration of indomethacin (400 micrograms) had no effect on the ACTH response to LPS, although it significantly suppressed febrile response. Small increases in plasma concentration of ACTH and significant fevers followed i.c.v. administration of prostaglandin E2 (2 micrograms) or F2 alpha (2 micrograms). I.v. administration of corticotropin releasing factor (CRF) antagonist [alpha-helical CRF (9-41) (200 micrograms/kg)] partly suppressed the ACTH increase induced in plasma by i.v. LPS. These results suggest that prostaglandins synthesized outside the blood-brain barrier play an important role in the ACTH response and that the mechanism for induction of the ACTH response is not exactly the same as that for the febrile response, although prostaglandins are involved in both responses.