Study on the possible entry of bacterial endotoxin and prostaglandin E2 into the central nervous system from the blood

Fever and hypothermia in systemic inflammation

Systemic inflammation-associated syndromes (e.g., sepsis and septic shock) often have high mortality and remain a challenge in emergency medicine. Systemic inflammation is usually accompanied by changes in body temperature: fever or hypothermia. In animal studies, systemic inflammation is often modeled by administering bacterial lipopolysaccharide, which triggers autonomic and behavioral thermoeffector responses and causes either fever or hypothermia, depending on the dose and ambient temperature. Fever and hypothermia are regulated changes of body temperature, which correspond to mild and severe forms of systemic inflammation, respectively. Mediators of fever and hypothermia are called endogenous pyrogens and cryogens; they are produced when the innate immune system recognizes an infectious pathogen. Upon an inflammatory challenge, hepatic and pulmonary macrophages (and later brain endothelial cells) start to release lipid mediators, of which prostaglandin (PG) E₂ plays the key role, and cytokines. Blood PGE₂ enters the brain and triggers fever. At later stages of fever, PGE₂ synthesized within the blood-brain barrier maintains fever. In both cases, PGE₂ is synthesized by cyclooxygenase-2 and microsomal PGE₂synthase-1. Mediators of hypothermia are not well established. Both fever and hypothermia are beneficial host defense responses. Based on evidence from studies in laboratory animals and clinical trials in humans, fever is beneficial for fighting mild infection. Based mainly on animal studies, hypothermia is beneficial in severe systemic inflammation and infection.

Mechanisms of fever production and lysis: lessons from experimental LPS fever

Fever is a cardinal symptom of infectious or inflammatory insults, but it can also arise from noninfectious causes. The fever-inducing agent that has been used most frequently in experimental studies designed to characterize the physiological, immunological and neuroendocrine processes and to identify the neuronal circuits that underlie the manifestation of the febrile response is lipopolysaccharide (LPS). Our knowledge of the mechanisms of fever production and lysis is largely based on this model. Fever is usually initiated in the periphery of the challenged host by the immediate activation of the innate immune system by LPS, specifically of the complement (C) cascade and Toll-like receptors. The first results in the immediate generation of the C component C5a and the subsequent rapid production of prostaglandin E2 (PGE2). The second, occurring after some delay, induces the further production of PGE2 by induction of its synthesizing enzymes and transcription and translation of proinflammatory cytokines. The Kupffer cells (Kc) of the liver seem to be essential for these initial processes. The subsequent transfer of the pyrogenic message from the periphery to the brain is achieved by neuronal and humoral mechanisms. These pathways subserve the genesis of early (neuronal signals) and late (humoral signals) phases of the characteristically biphasic febrile response to LPS. During the course of fever, counterinflammatory factors, "endogenous antipyretics," are elaborated peripherally and centrally to limit fever in strength and duration. The multiple interacting pro- and antipyretic signals and their mechanistic effects that underlie endotoxic fever are the subjects of this review.

A theoretical model of biochemical control engineering based on the relation between oestrogens/progestagens and prostaglandins

A biological complex organism is involuntarily guided from all sides by measure and regulation systems. The human being is such a complex organism. Many cyclical processes are simultaneously at work, making it unclear how and why which process takes place at which moment. Noticeable examples are the 28-day menstrual cycle and the 40-week pregnancy. The time of activation in the middle of the menstrual is fairly clear. Hormonal changes also occur in this period. Why the hormonal changes occur, and what their relationship is with the activation of the processes is unclear. That is also the case during pregnancies. What is it that determines that a pregnancy should last an average of 40 weeks? What causes the changes in a complicated pregnancy? What are those changes? Prostaglandin concentrations have been found to have some relationship with these changes, but the activation of these changes and how to examine them is unknown. Using an example from practical experience, this article illustrates what Horrobin and Manku already reported in 1977, namely, the properties of prostaglandin E1 and 6-keto pgF1α: reversal effect with elevated concentration. The properties described is exceptionally suitable for the time of activation in a biochemically regulated measure and regulation system. These properties can help explain the occurrence of physiological cycles. The known electronic saw-tooth wave has a biochemical analogue with this. This paper describes the presumed relationship between hormones and the accompanying prostaglandins with the hormone effects based on what is known regarding their concentrations progress. This relationship reveals the practical consequences of the experimentally found sensitivity of biochemical effects with regard to the accompanying prostaglandins. This paper shows how the theoretical relationship between effects of oestrogens and progestagens result in a curve that comprise observable aspects of the Basal Body Temperature Curve. The modulating and activating prostaglandins also affect local changes in blood circulation. These changes are visible on specific sites on the abdominal skin via viscerocutaneous reflex pathways. Changes in blood circulation at specific areas of the skin can be representative of pain. Pain that also frequently arises during activation processes. These changes can be seen and measured with non-contactual infrared thermography on the cutaneous surface, and moments of activation and pain can be determined.

Effects of central injection of anti-LPS antibody and blockade of TLR4 on GnRH/LH secretion during immunological stress in anestrous ewes

The present study was designed to examine the effect of intracerebroventricular (icv) administration of antilipopolysaccharide (LPS) antibody and blockade of Toll-like receptor 4 (TLR4) during immune stress induced by intravenous (iv) LPS injection on the gonadotropin-releasing hormone/luteinizing hormone (GnRH/LH) secretion in anestrous ewes. Injection of anti-LPS antibody and TLR4 blockade significantly (P < 0.01) reduced the LPS dependent lowering amount of GnRH mRNA in the median eminence (ME). Moreover, blockade of TLR4 caused restoration of LH-β transcription in the anterior pituitary decreased by the immune stress. However, there was no effect of this treatment on reduced LH release. The results of our study showed that the blockade of TLR4 receptor in the hypothalamus is not sufficient to unblock the release of LH suppressed by the immune/inflammatory challenges. This suggests that during inflammation the LH secretion could be inhibited directly at the pituitary level by peripheral factors such as proinflammatory cytokines and circulating endotoxin as well.

Role of Thymulin or Its Analogue as a New Analgesic Molecule

The thymic peptide thymulin is known for its immunomodulatory role. However, several recent reports have indicated that thymulin is capable of interacting directly and/or indirectly with the nervous system. One of the first lines of evidence of this interaction was obtained in a series of experiments showing the hyperalgesic actions of this peptide. We demonstrated that, at low doses (ng), local (intraplantar) or systemic (intraperitoneal) injections of thymulin resulted in hyperalgesia with an increase in proinflammatory mediators, and that this peptide could act directly on the afferent nerve terminals through prostaglandin-E2 (PGE2)-dependent mechanisms, thus forming a neuroimmune loop involving capsaicin-sensitive primary afferent fibers. In further experiments, systemic injections of relatively high doses (1-25 microg) of thymulin or of an analogue peptide (PAT) deprived of hyperalgesic effect, have been shown to reduce the inflammatory pain and the upregulated levels of cytokines induced by endotoxin (ET) injection. In addition, PAT treatment appeared to alleviate the sickness behavior (motor behavior and fever) induced by systemic inflammation. These effects could be attributed, at least partly, to the downregulation of proinflammatory mediators. Furthermore, when compared with the effects of other anti-inflammatory drugs, PAT exerted equal or even stronger analgesic effects, and at much lower concentrations. Subsequent experiments were designed to examine the effects of intracerebroventricular (i.c.v.) injections of thymulin on cerebral inflammation induced by i.c.v. injection of ET. Pretreatment with thymulin reduced, in a dose-dependent manner, the ET-induced hyperalgesia, and exerted differential effects on the upregulated levels of cytokines in different areas of the brain, suggesting a neuroprotective role for thymulin in the central nervous system (CNS). Preliminary results demonstrate that thymulin inhibits in the hippocampus the ET-induced nuclear activation of NF-kappaB, the transcription factor required for the expression of proinflammatory cytokines genes. Although the mechanism of action of these molecules is not totally elucidated, our results indicate a possible therapeutic use of thymulin or PAT as analgesic and anti-inflammatory drugs.

Endotoxin fever in granulocytopenic rats: evidence that brain cyclooxygenase-2 is more important than circulating prostaglandin E2

PGE(2) is a recognized mediator of many fevers, and cyclooxygenase (COX) is the major therapeutic target for antipyretic therapy. The source, as well as the site of action of PGE(2), as an endogenous pyrogen, is widely accepted as being central, but PGE(2) in the circulation, possibly from leukocytes, may also contribute to the development of fever. However, bacterial infections are important causes of high fever in patients receiving myelosuppressive chemotherapy, and such fevers persist despite the use of COX inhibitors. In the study reported here, the febrile response to bacterial LPS was measured in rats made leukopenic by cyclophosphamide. A striking increase in LPS fever occurred in these granulocytopenic rats when compared with febrile responses in normal animals. Unlike LPS fever in normal rats, fever in granulocytopenic rats was neither accompanied by an increase in blood PGE(2) nor inhibited by ibuprofen. Both leukopenic and normal rats showed LPS-induced COX-2-immunoreactivity in cells associated with brain blood vessels. Furthermore, LPS induced an increase of PGE(2) in cerebrospinal fluid. Induction of COX-2-expression and PGE(2) production was inhibited by ibuprofen in normal but not in leukopenic rats. Although the results presented are, in part, confirmatory, they add new information to this field and open a number of important questions as yet unresolved. Overall, the present results indicate that, in contrast to immunocompetent rats, leukocytes and/or other mechanisms other than PGE(2) are implicated in the mechanisms restricting and reducing the enhanced febrile response to endotoxin in immunosuppressed hosts.

Endotoxic fever: New concepts of its regulation suggest new approaches to its management

Endotoxic fever is regulated by endogenous factors that provide pro- and anti-pyretic signals at different points along the febrigenic pathway, from the periphery to the brain. Current evidence indicates that the febrile response to invading Gram-negative bacteria and their products is initiated upon their arrival in the liver via the circulation and their uptake by Kupffer cells (Kc). These pathogens activate the complement cascade on contact, hence generating complement component 5a. It, in turn, very rapidly stimulates Kc to release prostaglandin (PG)E2. Pyrogenic cytokines (TNF-alpha, etc.) are produced later and are no longer considered to be the immediate triggers of fever. The Kc-generated PGE2 either (1) may be transported by the bloodstream to the ventromedial preoptic-anterior hypothalamus (POA, the locus of the temperature-regulating center), presumptively diffusing into it and acting on thermoregulatory neurons; PGE2 is thus taken to be the final, central fever mediator. Or (2) it may activate hepatic vagal afferents projecting to the medulla oblongata, thence to the POA via the ventral noradrenergic bundle. Norepinephrine consequently secreted stimulates alpha1-adrenoceptors on thermoregulatory neurons, rapidly evoking an initial rise in core temperature (Tc) not associated with any change in POA PGE2; this neural, PGE2-independent signaling pathway is quicker than the blood-borne route. Elevated POA PGE2 and a secondary Tc rise occur later, consequent to alpha2 stimulation. Endogenous counter-regulatory factors are also elaborated peripherally and centrally at different points during the course of the febrile response; they are, therefore, anti-pyretic. These multiple interacting pathways are the subject of this review.

Kupffer cell-generated PGE2triggers the febrile response of guinea pigs to intravenously injected LPS

Because the onset of fever induced by intravenously (iv) injected bacterial endotoxic lipopolysaccharides (LPS) precedes the appearance in the bloodstream of pyrogenic cytokines, the presumptive peripheral triggers of the febrile response, we have postulated previously that, in their stead, PGE2could be the peripheral fever trigger because it appears in blood coincidentally with the initial body core temperature (Tc) rise. To test this hypothesis, we injected Salmonella enteritidis LPS (2 μg/kg body wt iv) into conscious guinea pigs and measured their plasma levels of LPS, PGE2, TNF-α, IL-1β, and IL-6 before and 15, 30, 60, 90, and 120 min after LPS administration; Tcwas monitored continuously. The animals were untreated or Kupffer cell (KC) depleted; the essential involvement of KCs in LPS fever was shown previously. LPS very promptly (<10 min) induced a rise of Tcthat was temporally correlated with the elevation of plasma PGE2. KC depletion prevented the Tcand plasma PGE2rises and slowed the clearance of LPS from the blood. TNF-α was not detectable in plasma until 30 min and in IL-1β and IL-6 until 60 min after LPS injection. KC depletion did not alter the times of appearance or magnitudes of rises of these cytokines, except TNF-α, the maximal level of which was increased approximately twofold in the KC-depleted animals. In a follow-up experiment, PGE2antiserum administered iv 10 min before LPS significantly attenuated the febrile response to LPS. Together, these results support the view that, in guinea pigs, PGE2rather than pyrogenic cytokines is generated by KCs in immediate response to iv LPS and triggers the febrile response.

Cytokines, PGE2 and endotoxic fever: a re-assessment

The innate immune system serves as the first line of host defense against the deleterious effects of invading infectious pathogens. Fever is the hallmark among the defense mechanisms evoked by the entry into the body of such pathogens. The conventional view of the steps that lead to fever production is that they begin with the biosynthesis of pyrogenic cytokines by mononuclear phagocytes stimulated by the pathogens, their release into the circulation and transport to the thermoregulatory center in the preoptic area (POA) of the anterior hypothalamus, and their induction there of cyclooxygenase (COX)-2-dependent prostaglandin (PG)E(2), the putative final mediator of the febrile response. But data accumulated over the past 5 years have gradually challenged this classical concept, due mostly to the temporal incompatibility of the newer findings with this concatenation of events. Thus, the former studies generally overlooked that the production of cytokines and the transduction of their pyrogenic signals into fever-mediating PGE(2) proceed at relatively slow rates, significantly slower certainly than the onset latency of fever produced by the i.v. injection of bacterial endotoxic lipopolysaccharides (LPS). Here, we review the conflicts between the earlier and the more recent findings and summarize new data that reconcile many of the contradictions. A unified model based on these data explicating the generation and maintenance of the febrile response is presented. It postulates that the steps in the production of LPS fever occur in the following sequence: the immediate activation by LPS of the complement (C) cascade, the stimulation by the anaphylatoxic C component C5a of Kupffer cells, their consequent, virtually instantaneous release of PGE(2), its excitation of hepatic vagal afferents, their transmission of the induced signals to the POA via the ventral noradrenergic bundle, and the activation by the thus, locally released norepinephrine (NE) of neural alpha(1)- and glial alpha(2)-adrenoceptors. The activation of the first causes an immediate, PGE(2)-independent rise in core temperature (T(c)) [the early phase of fever; an antioxidant-sensitive PGE(2) rise, however, accompanies this first phase], and of the second a delayed, PGE(2)-dependent T(c) rise [the late phase of fever]. Meanwhile-generated pyrogenic cytokines and their consequent upregulation of blood-brain barrier cells COX-2 also contribute to the latter rise. The consecutive steps that initiate the febrile response to LPS would now appear, therefore, to occur in an order different than conceived originally.

Albumin is not an irreplaceable carrier for amphipathic mediators of thermoregulatory responses to LPS: compensatory role of alpha1-acid glycoprotein

In view of the potential involvement of peripherally synthesized, circulating amphipathic mediators [such as platelet-activating factor (PAF) and prostaglandin E(2)] in the systemic inflammatory response to lipopolysaccharide (LPS), we hypothesized that transport of amphipaths by albumin is essential for conveying peripheral inflammatory signals to the brain. Our first specific aim was to test this hypothesis by studying LPS-induced fever and hypothermia in Nagase analbuminemic rats (NAR). NAR from two different colonies and normalbuminemic Sprague-Dawley rats were preimplanted with jugular catheters, and their febrile responses to a mild dose of LPS (10 microg/kg i.v.) at thermoneutrality and hypothermic responses to a high dose of LPS (500 microg/kg i.v.) in the cold were studied. NAR of both colonies developed normal febrile and hypothermic responses, thus suggesting that transport of amphipathic mediators by albumin is not indispensable for LPS signaling. Although alternative carrier proteins [such as alpha(1)-acid glycoprotein (AGP)] are known to assume transport functions of albumin in NAR, it is unknown whether inflammatory mediators are capable of inducing their actions when bound to alternative carriers. To test whether PAF, the most potent amphipathic pyrogen, causes fever when administered in an AGP-bound form was our second aim. Sprague-Dawley rats were preimplanted with jugular catheters, and their thermal responses to infusion of a 1:1 [PAF-AGP] complex (40 nmol/kg i.v.), AGP (40 nmol/kg i.v.), or various doses of free (aggregated) PAF were studied. The complex, but neither free PAF nor AGP, caused a high ( approximately 1.5 degrees C) fever with a short (< 10 min) latency. This is the first demonstration of a pyrogenic activity of AGP-bound PAF. We conclude that, in the absence of albumin, AGP and possibly other carriers participate in immune-to-brain signaling by binding and transporting amphipathic inflammatory mediators.

Tolerance develops to the effect of lipopolysaccharides on movement-evoked hyperalgesia when administered chronically by a systemic but not an intrathecal route

Single exposures to lipopolysaccharides (LPS) produce deep tissue pain in humans and cutaneous hyperalgesia in rodents. While tolerance develops to many effects of LPS, sensitization to hyperalgesia is documented after a single injection. To determine the effect of long-term exposure to LPS, we explored the chronic effect of LPS on movement-evoked pain using a new assay based on grip force in mice. We found that a single systemic injection of LPS (i.p. or s.c.) induced a dose-related decrease in forelimb grip force responses beginning 6-8 h after injection and peaking between 9 and 24 h. The consequence of LPS is likely hyperalgesia rather than weakness as these decreases were rapidly attenuated by either 10 mg/kg of morphine i.p. or 10 microg of morphine injected intrathecally (i.t.). Complete tolerance to this hyperalgesia developed after repeated injections of LPS at doses of 0.9 mg/kg i.p. or 5 mg/kg s.c. Tolerance began after a single injection and was fully developed after as few as four injections of 5 mg/kg of LPS delivered s.c. The concentration of circulating LPS 5 h after a single parenteral injection was less in LPS-tolerant mice than naïve controls, suggesting that tolerance may result from a more efficient clearance of LPS from the circulation. Injected i.t., LPS also induced hyperalgesia, however, tolerance did not develop to multiple injections by this route. There was no cross-tolerance between s.c. and i.t. injections of LPS. These data indicate that decreases in grip force are a sensitive measure of LPS-induced movement-evoked hyperalgesia and that tolerance develops to parenteral but not central hyperalgesic effects of LPS.

The effects of endogenous interleukin-1 bioactivity on locus coeruleus neurons in response to bacterial and viral substances

In a previous study, we found that microinjection of the cytokine interleukin-1 (IL-1) into the locus coeruleus (LC) increased the electrophysiological activity of LC neurons. To determine if endogenous IL-1 similarly affects the LC, brain IL-1 was induced with lipopolysaccharide (LPS), a substance derived from Gram-negative bacteria. LPS microinjected directly into the LC increased the activity of LC neurons in anesthetized rats, and this effect was blocked by microinfusion of the IL-1 receptor antagonist (IL-1RA) protein into the LC indicating the involvement of IL-1 receptors. Similarly, intraperitoneal (i.p.) LPS injection increased the activity of LC neurons in a dose- and time-related manner that was sensitive to IL-1RA. The change in the activity of LC neurons caused by a single i.p. injection of LPS was surprisingly long-lasting, and evolved over a period of at least 3 weeks. Other microbial substances-namely, peptidoglycan from Gram-positive bacteria and poly-inosine/poly-cytosine (poly(I)/(C)), which resembles RNA viruses-were used to determine the generality of the findings with LPS. Both i.p. peptidoglycan and poly(I)/(C) increased LC activity but with lesser efficacy than LPS. IL-1RA reversed the increase in the activity of LC neurons caused by i.p. peptidoglycan treatment; however, that caused by i.p. Poly(I)/(C) was not diminished by IL-1RA. Thus, the increased activity of LC neurons caused by LPS and peptidoglycan requires IL-1 receptor binding, suggesting the involvement of endogenously-produced IL-1. In contrast, poly(I)/(C) increased the activity of LC neurons but this did not critically involve IL-1 receptors in the LC.

Expression of genes controlling transport and catabolism of prostaglandin E2 in lipopolysaccharide fever

Prostaglandin (PG) E(2) is a principal downstream mediator of fever and other symptoms of systemic inflammation. Its inactivation occurs in peripheral tissues, primarily the lungs and liver, via carrier-mediated cellular uptake and enzymatic oxidation. We hypothesized that inactivation of PGE(2) is suppressed during LPS fever and that transcriptional downregulation of PGE(2) carriers and catabolizing enzymes contributes to this suppression. Fever was induced in inbred Wistar-Kyoto rats by intravenous LPS (50 microg/kg); the controls received saline. Samples of the liver, lungs, and hypothalamus were harvested 0, 0.5, 1.5, and 5 h postinjection. The expression of the two principal transmembrane PGE(2) carriers (PG transporter and multispecific organic anion transporter) and the two key PGE(2)-inactivating enzymes [15-hydroxy-PG dehydrogenase (15-PGDH) and carbonyl reductase] was quantified by RT-PCR. All four genes of interest were downregulated in peripheral tissues (but not the brain) during fever. Most remarkably, the expression of hepatic 15-PGDH was decreased 26-fold 5 h post-LPS, whereas expression of pulmonary 15-PGDH was downregulated (as much as 18-fold) throughout the entire febrile course. The transcriptional downregulation of several proteins involved in PGE(2) inactivation, first reported here, is an unrecognized mechanism of systemic inflammation. By increasing the blood-brain gradient of PGE(2), this mechanism likely facilitates penetration of PGE(2) into the brain and prevents its elimination from the brain.

Potent analgesic and anti-inflammatory actions of a novel thymulin-related peptide in the rat

1. The present study examines the effect of PAT (peptide analogue of thymulin) in two rat models of inflammatory hyperalgesia induced by either i.pl. (1.25 microg in 50 microl saline) or i.p. (50 microg in 100 microl) injections of endotoxin ET. 2. Pretreatment with PAT (1, 5 or 25 microg in 100 microl saline, i.p.) decreased, in a dose dependent manner, both mechanical hyperalgesia, determined by the paw pressure (PP) test and thermal hyperalgesia determined by the hot plate (HP), the paw immersion (PI) and the tail flick (TF) tests. 3. Compared to the tripeptides K(D)PT and K(D)PV, known to antagonize interleukin (IL)-1beta or IL-1beta and PGE(2) mechanisms, PAT, at lower dosages, exerted stronger anti-hyperalgesic effects. 4. When compared with the effect of a steroidal (dexamethasone) and a non-steroidal (indomethacin) anti-inflammatory drugs (NSAID), PAT demonstrated equal analgesic actions. 5. Pretreatment with PAT, reduced significantly the increased concentration of IL-1beta, IL-6, TNF-alpha and NGF due to i.pl. injection of ET. 6. Injection of i.p. ET produced sickness behaviour characterized by hyperalgesia and fever. Pretreatment with PAT prevented the hyperalgesia and maintained the body temperature within the normal range and was accompanied by a down-regulation of the levels of pro-inflammatory cytokines and PGE(2) in the liver. 7. PAT, in all doses used, did not result in any evident changes in the physiological parameters or in the normal behaviour of the rats. 8. The anti-hyperalgesic and anti-inflammatory effects of PAT can be attributed, at least partially, to the down-regulation of pro-inflammatory mediators.

Mechanism of action of acetaminophen: is there a cyclooxygenase 3?

Acetaminophen, also known as paracetamol, is a nonsteroidal anti-inflammatory drug with potent antipyretic and analgesic actions but with very weak anti-inflammatory activity. When administered to humans, it reduces levels of prostaglandin metabolites in urine but does not reduce synthesis of prostaglandins by blood platelets or by the stomach mucosa. Because acetaminophen is a weak inhibitor in vitro of both cyclooxygenase (COX)-1 and COX-2, the possibility exists that it inhibits a so far unidentified form of COX, perhaps COX-3. In animal studies, COX enzymes in homogenates of different tissues vary in sensitivity to the inhibitory action of acetaminophen. This may be evidence that there are >2 isoforms of the enzyme. Recently, a variant of COX-2 induced with high concentrations of nonsteroidal anti-inflammatory drugs was shown to be highly sensitive to inhibition by acetaminophen. Therefore COX-3 may be a product of the same gene that encodes COX-2, but have different molecular characteristics.

Effects of the inhibition of cyclo-oxygenase 1 or 2 or 5-lipoxygenase on the activation of the hypothalamic-pituitary-adrenal axis induced by interleukin-1beta in the male Rat

The limited entry of interleukin-1beta (IL-1beta) into the central nervous system has led to the hypothesis that IL-1beta acts, through IL-1beta receptors located notably on endothelial cells, on the release of prostaglandins which in turn stimulate the hypothalamic-pituitary-adrenal (HPA) axis. We used cyclo-oxygenase-1 (COX-1) and cyclo-oxygenase-2 (COX-2) and 5-lipoxygenase (5-LOX) inhibitors, before the injection of IL-1beta, to explore the role of arachidonic acid metabolic pathways on HPA axis activation. Adult male rats were i.m injected 20 min before i.p injection of IL-1beta, with (i): a COX-1/COX-2 inhibitor (ketoprofen); (ii) a COX-2 selective inhibitor (NS 398); or (iii) a 5-LOX inhibitor (BW A4C). Following this, rats were killed 90 min after i.p. IL-1beta injection and analysis for plasma adrenocorticotropic hormone (ACTH) and corticosterone concentrations and determination of anterior pituitary pro-opio melanocortin (POMC) gene transcription was conducted. Administration of the COX-1/COX-2 inhibitor led to a complete blockage of ACTH and corticosterone secretion and POMC gene transcription. The COX-2 inhibitor led to a complete blockade of ACTH secretion and POMC gene transcription but had no effect on corticosterone secretion. The 5-LOX inhibitor had no significant effect on any parameter. These results demonstrate the crucial role of eicosanoid pathways in mediating the stimulation of the HPA axis induced by IL-1beta. Moreover, we found a clear dissociation of the effect of the blockage of COXs upon ACTH and corticosterone secretion, suggesting that IL-1beta may act at the brain as well as at the adrenal cortex to stimulate the secretion of corticosterone.

Role of central glutamate receptors, nitric oxide and soluble guanylyl cyclase in the inhibition by endotoxin of rat gastric acid secretion

1. This study examines the role of a central pathway involving glutamate receptors, nitric oxide (NO) and cyclic GMP in the acute inhibitory effects of low doses of peripheral endotoxin on pentagastrin-stimulated acid production. 2. Vagotomy or intracisternal (i.c.) microinjections of the NO-inhibitor, N(G)-nitro-L-arginine methyl esther (L-NAME; 200 microg rat(-1)) restored acid secretory responses in endotoxin (10 microg kg(-1), i.v.)-treated rats. 3. The acid-inhibitory effect of i.v. endotoxin (10 microg kg(-1), i.v.) was prevented by prior i.c. administration of the NMDA receptor antagonists, dizocilpine maleate (MK-801; 10 nmol rat(-1)) and D-2-amino-5-phosphono-valeric acid (AP-5; 20 nmol rat(-1)), or the AMPA/kainate antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX; 10 nmol rat(-1)). However, the competitive metabotropic glutamate receptor antagonist (+)-alpha-methyl-4-carboxyphenylglycine (MCPG; 20 - 1000 nmol rat(-1)) did not antagonize the effects of endotoxin. 4. I.c. administration of L-glutamate (0.1 nmol rat(-1)) inhibited pentagastrin-stimulated gastric acid secretion. Coadministration with L-NAME (200 microg rat(-1)) prevented the inhibition of gastric acid secretion by the aminoacid. 5. I.c. administration of 1H-[1,2, 4]Oxazodiolo[4,3-a]quinoxalin-1-one (ODQ; 100 nmol rat(-1)), a soluble guanylyl cyclase (sGC) blocker, reversed the hyposecretory effect of endotoxin. 6. I.c. administration of the cyclic GMP analogue 8-Bromoguanosine-3,5-cyclic monophosphate (8-Br-cGMP; 100 - 300 nmol rat(-1)) reduced gastric acid production in a dose-dependent manner. 7. We conclude that central NMDA and AMPA/kainate receptors are involved in the acid inhibitory effect of peripherally administered endotoxin. This central pathway involves synthesis of NO, which acts on the enzyme sGC.

Cytokine and adrenal axis responses to endotoxin

The push-pull cannula (PPC) technique was applied to examine the kinetics of in vivo concentration changes in male rat brain extracellular fluid (ECF) of endogenous interleukin-1beta (IL-1beta) and corticotrophin releasing hormone (CRH) after a peripheral injection of lipopolysaccharide (LPS) (25 microg/100 g b.wt. intravenously). In addition, IL-1beta, adrenocorticotropin hormone (ACTH) and corticosterone concentrations in plasma were also measured at selected intervals after LPS challenge. Administration of LPS resulted in a progressive increase in the concentrations of IL-1beta in brain hypothalamic ECF. A significant increase from the zero time mean value of 77+/-10 to 393+/-88 pg/ml at the 15-min interval was recorded. The increase in IL-1beta concentration in hypothalamic ECF reached a peak of 883+/-237 pg/ml at 30 min post-LPS. CRH concentration in the same hypothalamic ECF was 41+/-17 pg/ml at time zero, 97+/-15 pg/ml at 15 min and at 30 min was significantly increased (215+/-56 pg/ml). A time course of significant increases at 30 min in plasma concentrations of IL-1beta, ACTH and corticosterone was also recorded in the same animals described above. The data show that a peripherally administered LPS bolus elicited an early (over 15 min post-injection) increase in brain ECF IL-1beta concentration; additional significant increases in hormones released from the hypothalamic-pituitary-adrenal (HPA) axis were recorded at 30 min post-LPS injection. These observations support the concept of an early change in hypothalamic ECF concentration of IL-1beta preceding LPS-induced activation of the HPA axis.

Blood-borne, albumin-bound prostaglandin E2 may be involved in fever

Although the involvement of blood-borne PGE2 in fever has been hypothesized by several authors and has substantial experimental support, the current literature often rejects this hypothesis because several attempts to induce fever by a peripheral PGE2 failed. However, it is usually ignored that the amphipathic molecules of PGE2 are readily self-associating and that such an aggregation could have prevented the peripherally administered PGE2 (free form) from expressing its pyrogenic activity, thus leading to false negative results. To ensure disaggregation of PGE2, we prepared its complex within a carrier protein, human serum albumin (HSA). HSA was purified with activated charcoal and polymixin B-polyacrylamide gel and incubated with PGE2 for 1 h at 37 degrees C. Adult Chinchilla rabbits were injected intravenously with PGE2-HSA complex in either the higher (75 micrograms/kg PGE2:30 mg/kg HSA) or the lower (15 micrograms/kg:6 mg/kg) dose, and the rectal temperature (Tr) was measured. In the controls, either the ligand alone or the carrier alone was administered. At the higher dose, neither free PGE2 nor albumin alone was pyrogenic, whereas the PGE2-HSA complex produced a fever characterized by a short latency (<10 min) and a maximal Tr rise of 0.7 +/- 0.2 degrees C. At the lower dose, none of the substances affected the Tr. This study demonstrates a marked pyrogenic activity of the intravenous PGE2-HSA, but not of the free PGE2. Administration of a preformed complex may be more physiologically relevant than administration of the free ligand because of the ligand's disaggregation, protection from enzymatic degradation, and facilitated delivery to targets. Our study supports the hypothesis that peripheral PGE2 is involved in fever genesis.

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.

Vagotomy attenuates but does not prevent the somnogenic and febrile effects of lipopolysaccharide in rats

The role of the vagus nerve in the somnogenic and pyrogenic effects of lipopolysaccharide (LPS) was studied in rats. Control rats (n = 8) and rats subjected to bilateral subdiaphragmal vagotomy (VX; n = 9) were injected with 100 micrograms/kg i.p. LPS at the beginning of the dark period. Sleep and brain temperature (Tbr) were recorded for 23 h after the injections. LPS caused increases in non-rapid eye movement sleep (NREMS) for 12 h after the injection in control rats. Sleep intensity, as indicated by the slow-wave activity (SWA) of the electroencephalogram during NREMS, was suppressed. LPS elicited biphasic Tbr responses: an initial hypothermia was followed by increases in Tbr that lasted for approximately 20 h. In vagotomized rats, the NREMS responses to LPS were blunted. The magnitude of the LPS-induced NREMS increases was about one-half of that seen in control rats, and these sleep responses lasted only for 6 h. LPS did not affect SWA in VX animals. VX completely abolished the hypothermic responses to LPS and shortened the duration of the hyperthermia. The results suggest that the subdiaphragmal vagi play an important, but not exclusive, role in the somnogenic and pyrogenic actions of intraperitoneally injected LPS.

Whole body metabolic responses to brain trauma in the rat

Increases in metabolic rate reported in head-injured patients can contribute to increases in respiratory demand, raised body temperature, and host body wasting (cachexia). The objective of the present study was to quantify the metabolic responses to brain trauma in the rat and investigate the underlying mechanisms. Lateral fluid-percussion (FP) injury (applied cortical pressure 1.6-1.8 atm) in the rat resulted in consistent and reproducible cortical brain lesions (44 +/- 6 mm3). Body weight and food intake were reduced significantly 24 h after brain trauma compared to sham-operated (7 and 49%,p < 0.01) and control animals (14 and 65%,p < 0.001), respectively. Resting oxygen consumption (V(O2), measured at 24 degrees C) was increased significantly, by 9-16% above sham-operated, and 14-26% above control animals for 2-7 h after brain trauma (p < 0.05), but V(O2) was not raised thereafter (24-72 h) and colonic temperature was not changed. Raising the ambient temperature from 24 degrees C to 28 degrees C significantly reduced the hypermetabolism of brain-injured rats compared to sham-operated controls. Injection of the beta-adrenoceptor antagonist propranolol (10 mg/kg, i.p.) completely abolished the rise in metabolic rate of brain-injured rats, and reduced significantly the rise in metabolic rate of the sham-operated animals (26%, p < 0.01 and 11%, p < 0.05; respectively). Systemic injection of the cyclo-oxygenase inhibitor indomethacin (1 mg/kg, i.p.) significantly attenuated (by 11%,p < 0.01), but did not completely abolish the hypermetabolism of brain-injured animals. Lateral FP injury in the rat causes a significant cachexia. Weight loss is due to hypophagia, and an increase in energy expenditure, which is mediated by sympathetic activation of thermogenesis and in part by prostaglandins.

Involvement of prostaglandins in cachexia induced by T-cell leukemia in the rat

We have previously demonstrated that experimentally induced T-cell leukemia in the rat results in a rapid and severe cachexia. This weight loss is largely due to a reduction in food intake, but is also accompanied by inappropriately high rates of energy expenditure. Increases in resting oxygen consumption (VO2) of 25% to 35% above the levels of pair-fed animals were observed over the period of weight loss. The present study investigated the possible involvement of prostaglandins in the cachexia induced by T-cell leukemia in the rat. Acute systemic injection of the cyclo-oxygenase inhibitors (indomethacin 1 mg/kg or flurbiprofen 1 mg/kg intraperitoneally [IP]) significantly reduced (by 14% and 10%, respectively) the increase in metabolic rate and also reversed the elevated body temperature of leukemic animals. Intracerebroventricular (ICV) injection of indomethacin (0.2 mg/kg) had only modest effects on the increase in temperature or hypermetabolism of leukemic animals. Long-term daily injection of indomethacin or flurbiprofen (1 mg/kg/d IP) had no significant effect on food intake or body weight of leukemic animals, and neither treatment significantly affected disease status. Indomethacin significantly reduced the decline in epididymal fat pad weight of leukemic animals. These data indicate that prostaglandins, produced peripherally, are involved in the acute hypermetabolism associated with T-cell leukemia, but have little or no effect on the hypophagia or body weight loss of leukemic rats.

A comparative study of adrenocorticotropin-releasing activity of prostaglandins E1, E2, F2 alpha and D2 in the rat

There is substantial evidence to indicate that prostaglandin (PG) E2 exerts a stimulatory effect on the hypothalamo-pituitary-adrenal axis in rodents. However, little is known regarding the possibility that other PGs play a similar role in regulating the endocrine axis. Therefore, in this study we compared the effects of intravenous administration of PGs E1, E2, F2 alpha and D2 on adrenocorticotropin (ACTH) secretion in conscious male rats. Each PG was administered at two doses of 0.1 and 1.0 mg/kg body weight, and blood samples were collected sequentially up to 120 min postinjection. Although PGD2 was without effect on ACTH secretion at either dose, PGs E1, E2 and F2 alpha all significantly stimulated the hormonal response at both doses. Interestingly, PGs E1, E2 and F2 alpha were largely equipotent in stimulating ACTH release. To the best of our knowledge, this is the first study to demonstrate significant ACTH-releasing activity of intravenously administered PGs E1 and F2 alpha in the rat. These results suggest that PGE2 might not be the only prostanoid playing a role in regulating the hypothalamo-pituitary-adrenal axis and, thus, multiple PGs may be involved in the endocrine axis.

Endothelial cells of the rat brain vasculature express cyclooxygenase-2 mRNA in response to systemic interleukin-1 beta: a possible site of prostaglandin synthesis responsible for fever

We previously showed that intraperitoneal injection of lipopolysaccharide induced cyclooxygenase-2 (COX-2) mRNA in as yet unidentified cells of blood vessels and leptomeninges in the rat brain and proposed a possible role of these cells as the source of prostaglandin E2 in the genesis of fever (Cao et al., Brain Res., 697 (1995) 187-196). In the present study, to proceed further with this line of research, we addressed the following two questions: first, does a pyrogenic dose of interleukin-1 beta (IL-1 beta), an endogenous pyrogen, induce COX-2 mRNA in the brain blood vessels and leptomeninges? Secondly, if it does, what type of cells are positive for COX-2 mRNA? Intraperitoneal injection of recombinant human IL-1 beta (30 micrograms/kg) induced fever in rats and an in situ hybridization study revealed that faint but significant COX-2 mRNA signals appeared in the blood vessels and leptomeninges at 1.5 h after the injection (the early rising phase of fever). The mRNA signals increased in number and intensity at 4 h (early plateau phase), decreased at 6.5 h (early recovery phase), and completely disappeared by 10 h after the injection (late recovery phase). The COX-2 mRNA positive cells in the blood vessels were likely to be the endothelial cells since the corresponding cells in the adjacent mirror-imaged section also expressed mRNAs for intracellular adhesion molecule-1 and the type-I interleukin-1 receptor, although those in the leptomeninges still remained unidentified. These results imply that circulating IL-1 beta acts on its receptor on the endothelial cells of the brain vasculature to induce COX-2 mRNA, which is possibly responsible for the elevated level of PGE2 seen during fever.

Hypothalamic prostaglandin E2 during lipopolysaccharide-induced fever in guinea pigs

Prostaglandin E2 (PGE2) is postulated to be a central mediator of fever. It is generally believed that it is produced in the preoptic area of the anterior hypothalamus (POA) because, among other evidence, its level increases both in the third ventricle and in the POA in response to pyrogens. However, lately, the question has arisen whether PGE2 might, in fact, be formed outside of the brain substance and then penetrate it, in particular through the organum vasculosum laminae terminalis. If produced outside the brain substance, the peripheral blockade of its synthesis should prevent lipopolysaccharides (LPS)-induced fever, whereas the intracarotid infusion of PGE2 should produce an increase in core temperature (T(C)) as well as in preoptic PGE2. To verify this hypothesis, continuous measurements of T(C) and preoptic PGE2 levels were made in conscious guinea pigs administered the PGE2 synthase inhibitor, indomethacin (10 or 50 mg/kg, im) 30 min before S. enteritidis LPS (2 mu g/kg, iv) or before PGE2 microdialyzed into the POA (1 mu g/mu l at 2 mu g/min for 2.5 h) and during PGE2 infused into a carotid artery (1 mu g and 10 mu g/mu l at 2 mu g/min for 1 h). LPS induced a biphasic 1.4 degrees C fever that was consistently associated with an increase in the level of PGE2 in the POA. Indomethacin at 10 mg/kg attenuated the course of the LPS-induced fever and prevented the associated increase in preoptic PGE2 for 90 min after fever onset; thereafter, PGE2 was significantly reduced by comparison with controls. Indomethacin at 50 mg/kg completely abolished both the fever and the increased levels of PGE2 in the POA; the fever induced by PGE2 microdialyzed into the POA was not affected by indomethacin pretreatment The intracarotid infusion of PGE2 produced T(C) falls and no increase in preoptic PGE2 levels. The indomethacin-induced blockade of fever and inhibition of the associated increase in preoptic PGE2 levels further substantiates the presumptive link between PGE2 in the POA and fever caused by LPS. The failure of exogenous PGE2 infusion to induce increases in T(C) and preoptic PGE2 levels excludes the possibility that PGE2 formed outside of the brain penetrates the POA and induces fever. Thus, in guinea pigs, the PGE2 associated with LPS-induced fever may be synthesized in the POA.

Induction by lipopolysaccharide of cyclooxygenase-2 mRNA in rat brain; its possible role in the febrile response

Cyclooxygenase 2 (COX-2) is a newly discovered isoform of cyclooxygenase that is inducible by lipopolysaccharide (LPS) or cytokines. This enzyme is considered to play a major role in inflammatory processes by catalyzing the production of prostaglandins. In the present study, induction of COX-2 mRNA in the rat brain by intraperitoneal injection of LPS was studied by the in situ hybridization technique with special attention paid to timing and sites of induction along with the time course of fever. In situ hybridization was carried out on sections of rat brain, 1 h (latent phase), 2.5 h (maximally febrile phase), 4 h (plateau phase), and 7 h (recovery phase) after the LPS injection, as well as on those from the brains of untreated and saline-injected rats. Injection of LPS induced COX-2 mRNA in the brain in two different constituents: neuronal cells and non-parenchymal cells of the blood vessels and leptomeninges. Induction in the neuronal cells was restricted to some telencephalic areas where the COX-2 mRNA signal was also detected in control animals. The signal was maximally enhanced by 50 to 80% over the basal level 1 h after LPS injection. The COX-2 mRNA signal was hardly detectable in neuronal and glial cells in other brain regions, including the preoptic area, either in control or LPS-injected rats. Strong COX-2 mRNA signals, however, appeared in the inner surface of blood vessels and the leptomeninges over the entire brain, including the preoptic area and its vicinity. The signals were not detectable in the brains of control rats and were most intense in the brains of rats treated with LPS for 2.5 h or 4 h. These results demonstrate that two major cell groups in the brain, neuronal cells and non-parenchymal cells, are responsible for the enhanced production of prostaglandins after systemic LPS treatment. Considering the site and timing of induction, we propose a possible role for blood vessels and leptomeninges as the source of prostaglandin E2 in the genesis of fever.

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.

Pyrogenic stimulation of vascular resistance in conscious sheep

Increased arterial blood pressure following a pyrogenic reaction has been reported in previous studies, however the mechanism of this hypertension has not been examined in detail. The present study investigated the effects of both intravenous (IV) and intracerebroventricular (ICV) injection of lipopolysaccharide (LPS) from E. coli on body temperature (Tb), mean arterial pressure (MAP), heart rate (HR), cardiac output (CO), calculated total peripheral resistance (CTPR), stroke volume (SV) and plasma levels of adrenocorticotropin (ACTH) and arginine vasopressin (AVP) in conscious, chronically instrumented sheep. IV injection of LPS (1 microgram) increased Tb in a biphasic manner from 38.7 +/- 0.1 to 39.5 +/- 0.2 degrees C after 50 min and to 39.9 +/- 0.2 degrees C after 130 min, and MAP increased biphasically from 64 +/- 1 to 70 +/- 4 mmHg after 40 min and to 78 +/- 3 mmHg after 130 min. CO initially decreased from 4.4 +/- 0.1 to 3.5 +/- 0.1 after 40 min followed by a secondary rise to 4.8 +/- 0.1 l/min after 100 min. This occurred together with a large, biphasic increase in CTPR from 14.5 +/- 1.0 to 22.0 +/- 2.0 mmHg/l/min at 40 min, and to 18.1 +/- 0.1 mmHg/l/min at 120 min. HR increased from 68 +/- 4 to 97 +/- 4 b/min and SV decreased from 65 +/- 2 to 41 +/- 4 ml/beat during the first phase of activation. Plasma ACTH increased from 22 +/- 9 to 1043 +/- 175 pg/ml after 80 min, and plasma AVP increased from 0.7 +/- 0.2 to 12 +/- 4.0 pg/ml after 60 min. ICV injection of LPS produced a long-lasting increase in Tb and MAP, but had no effect on HR or plasma AVP. Plasma ACTH increased from 30 +/- 12 to 427 +/- 110 pg/ml. These changes suggest that intravenous pyrogenic infection produces a potent vasoconstrictor action in sheep to increase blood pressure, possibly mediated by the actions of AVP within the CNS, or other pyrogenically released vasoconstrictor factors. Furthermore, the duration of activation of the cardiovascular system following peripheral and central LPS administration is different, which together with the contrasting effects on ACTH and AVP, indicate the involvement of several hypertensive mechanisms.

Effects of arterial hypoxia and beta-adrenoceptor blockade on cerebral blood flow and oxygen uptake following E. coli endotoxin in dogs

Earlier studies in normoxia have shown that an endotoxin injection in dogs leads to an increase in cerebral metabolic rate of oxygen (CMRo2), a decrease in cerebral blood flow (CBF) and increased concentrations of monoamines in blood and cerebrospinal fluid (CSF). In animals pretreated with propranolol (PPL) the CMRo2 increase was abolished and thus beta-adrenoceptor mediated. Arterial hypoxia normally increases CBF without any influence on CMRo2. The aim of this study was to investigate the effects of moderate arterial hypoxia on CBF, CMRo2 and catecholamine concentrations in blood and CSF after endotoxin with and without pretreatment with PPL. Three groups of dogs were studied. Group 1: Six animals were subjected to arterial hypoxia without any other intervention. Group 2: Six animals were given an endotoxin injection (E. coli lipopolysaccharide O 111: B 4), before the induction of hypoxia. Group 3: Eight animals were pretreated with PPL per os, 12.5 mg.kg-1 twice a day for one week before the experiments, and the effects of arterial hypoxia were studied both before and after an intravenous injection of endotoxin. Two levels of hypoxia were studied; oxygen saturation in arterial blood aiming at 75 and 50%. Endotoxin was given intravenously in a dose of 1 mg.kg-1 bodyweight over a 5 minute period. After an endotoxin injection, the response to arterial hypoxia was an increase in CMRo2, in contrast to the unchanged CMRo2 without endotoxin. After pretreatment with PPL the increase in CMRo2 after endotoxin was prevented. The CBF reaction to hypoxia was uniformly an increase.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of propranolol pretreatment on cerebral blood flow, oxygen uptake and catecholamines during metabolic acidosis following E. coli endotoxin in dogs

After an intravenous injection of E. coli endotoxin in dogs a decrease in cerebral blood flow (CBF) and an increase in cerebral metabolic rate of oxygen (CMRo2) have been shown to occur. In metabolic acidosis following endotoxin CMRo2 increased with decreasing pH. A possible explanation for the increased CMRo2 after endotoxin and metabolic acidosis seems to be a damage of the blood-brain barrier (BBB) by endotoxin. This gives possibilities for a leakage of hydrogen ions and circulating monoamines from the blood to the brain, thus affecting the cerebral blood flow and metabolism. The effects of an E. coli endotoxin injection on CBF and CMRo2 during metabolic acidosis and beta-adrenoceptor blockade were studied in eight anaesthetized dogs. All the dogs were pretreated with propranolol (PPL), per os 12.5 mg.kg-1 twice a day for one week. Metabolic acidosis (pH 7.01-7.30) was achieved by an intravenous infusion of hydrochloric acid. Endotoxin E. coli lipopolysaccharide O 111:B 4 was given as an intravenous injection of 1 mg.kg-1 bodyweight over a 5 min period. Another five animals, published earlier, with the same experimental protocol but without PPL, constituted a control group. After endotoxin no increase in CMRo2 or CBF was observed with increasing acidosis in the PPL-group. In the control group, after endotoxin, both CBF and CMRo2 increased with decreasing pH. This resulted in a significant difference in both CBF and CMRo2 between the groups in the pH range 7.01-7.15. The present results indicate that the increase in CMRo2 and CBF with metabolic acidodis in endotoxinaemia is mediated via beta-adrenoceptors.

Characterization of cytokine-induced hyperalgesia

Agents which induce symptoms of illness, such as lipopolysaccharide (LPS), cause diverse effects including hyperalgesia. While previous studies have examined central pathways mediating LPS hyperalgesia, the initial steps in activating this system remain unknown. Since LPS induces the release of various cytokines and eicosinoids from immune cells, the present series of experiments examined the potential involvement of these substances in LPS hyperalgesia. This work demonstrates that: (a) Interleukin-1 beta (IL-1 beta) can produce hyperalgesia following either intraperitoneal or intracerebroventricular injection. In contrast, IL-1 beta delivered intrathecally did not affect pain responsivity. (b) Liver macrophages (Kupffer cells) appear to be critically involved, and relay signals to the brain via hepatic vagal afferents. (c) Both IL-1 beta and tumor necrosis factor appear to be critical mediators of LPS hyperalgesia. In contrast, prostaglandins do not appear to be involved. Taken together, these studies suggest that substances classically thought of as products of the immune system may dynamically enhance pain responsivity via actions either on the hepatic vagus or at central sites.

Does endogenous peripheral arginine vasopressin have a role in the febrile responses of conscious rabbits?

1. The actions of peripheral arginine vasopressin (AVP) on the febrile responses of conscious rabbits induced by peripherally administered polyinosinic:polycytidylic acid (poly(I).poly(C)) have been studied using an AVP V1 receptor antagonist ([deamino-Pen1, O-Me-Tyr2, Arg8]-vasopressin). 2. Temperature responses were monitored continuously using rectal thermistor probes. Test substances were administered intravenously (i.v.). Blood samples were taken at timed intervals from a marginal ear vein and plasma PGE2 and PGF2 alpha levels determined by radioimmunoassay. 3. Poly(I).poly(C) (2.5 micrograms/kg) stimulated a reproducible biphasic rise in body temperature with a lag phase of 45-60 min and peaks at 90 and 225 min. The febrile response was accompanied by a 5-fold rise in circulating immunoreactive (ir) PGE2, which peaked after 90 min and remained elevated up to 300 min. Poly(I).poly(C) also stimulated a 2.5-fold rise in circulating irPGF2 alpha, which peaked after 150 min and was followed by a return to basal levels after 300 min. 4. The overall magnitude of the febrile response to poly(I).poly(C) (2.5 micrograms/kg, i.v.) was significantly antagonized by the AVP V1 receptor antagonist (250 micrograms/kg, i.v.) administered 5 min prior to the pyrogen. 5. The irPGE2 response to poly(I).poly(C) (2.5 micrograms/kg, i.v.) was significantly antagonized by the AVP V1 receptor antagonist (250 micrograms/kg, i.v.) administered 5 min prior to the pyrogen. The irPGF2 alpha response was only reduced at the peak 150 min time point measurement. 6. In conclusion, these results show a modulatory role for a peripherally administered AVP V1 antagonist in the febrile responses to poly(I).poly(C), suggesting a possible propyretic role for endogenous peripheral AVP. This modulatory role appears to be mediated via actions on prostaglandin E2.

Fever: causes and consequences

The present review distinguishes pathogenic, neurogenic, and psychogenic fever, but focuses largely on pathogenic fever, the hallmark of infectious disease. The data presented show that a complex cascade of events underlies pathogenic fever, which in broad outline - and with frank disregard of contradictory data - can be described as follows. An invading microorganism releases endotoxin that stimulates macrophages to synthesize a variety of pyrogenic compounds called cytokines. Carried in blood, these cytokines reach the perivascular spaces of the organum vasculosum laminae terminalis (OVLT) and other regions near the brain where they promote the synthesis and release of prostaglandin (PGE2). This prostaglandin then penetrates the blood-brain barrier to evoke the autonomic and behavioral responses characteristic of fever. But then once expressed, fever does not continue unchecked; endogenous antipyretics likely act on the septum to limit the rise in body temperature. The present review also examines fever-resistance in neonates, the blunting of fever in the aged, and the behaviorally induced rise in body temperature following infection in ectotherms. And finally it takes up the question of whether fever enhances immune responsiveness, and through such enhancement contributes to host survival.

Inhibition of the thermogenic and pyrogenic responses to interleukin-1 beta in the rat by dietary N-3 fatty acid supplementation

The thermogenic (increase in oxygen consumption, VO2) and pyrogenic (Tc) responses to the cytokine interleukin-1 beta (IL-1 beta) were studied in rats fed a n-3 fatty acid supplemented diet (8.75% n-3 fatty acids/kg diet). 4-6 weeks after commencing the diets, the n-3 supplemented rats exhibited reduced pyrogenic (0.5 +/- 0.1 degrees C versus 1.1 +/- 0.2 degrees C in control animals) and thermogenic (9 +/- 3% versus 22 +/- 6% in control animals) responses to intraperitoneal (i.p.) injection of IL-1 beta (1 micrograms/rat). However, responses to centrally administered IL-1 beta (5ng intracerebroventricular (i.c.v.)) were similar in both groups at this time. After 8-9 weeks of supplementation, n-3 supplemented animals exhibited attenuated responses to both ip IL-1 beta (VO2 responses reduced by 68% and Tc by 0.8 degrees C) and also i.c.v. IL-1 beta (VO2 responses reduced by 56% and Tc by 0.7 degrees C). N-3 supplementation did not, however, influence the thermogenic capacity of these animals since responses to noradrenaline were similar in control and n-3 fed animals (50% increase in VO2). These findings demonstrate that n-3 supplementation modifies the pyrogenic and thermogenic responses to IL-1 beta, probably via changes in eicosanoid metabolism. Modification of central responses to IL-1 are delayed compared to the effects of peripheral administration indicating separate mechanisms of IL-1 on fever and thermogenesis in the brain and the periphery.

A possible role for endogenous peripheral corticotrophin-releasing factor-41 in the febrile response of conscious rabbits

1. The actions of peripheral corticotrophin-releasing factor-41 (CRF-41) on the febrile responses of conscious rabbits induced by peripherally administered polyinosinic.polycytidylic acid (poly(I).poly(C)) have been studied using a CRF-41 receptor antagonist (alpha-helical CRF(9-41) and anti-CRF-41 monoclonal antibodies. 2. Temperature responses were monitored continuously using rectal thermistor probes. Test substances were administered intravenously (i.v.), or for central CRF-41 antagonism experiments, via an indwelling third ventricle cannula (I.C.V.). Blood samples were taken at time intervals from a marginal ear vein and plasma cortisol levels determined by radioimmunoassay. 3. Poly(I).poly(C) (2.5 micrograms/kg) stimulated a reproducible biphasic rise in body temperature with a lag phase of 45-60 min and peaks at 90 and 225 min. 4. The febrile response to poly(I).poly(C) (2.5 micrograms/kg I.V.) was antagonized by blockade of peripheral CRF-41 actions using either monoclonal anti-CRF-41 antibodies (2.5 mg/kg i.v.) or the CRF-41 receptor antagonist (alpha-helical CRF(9-41); 25 micrograms/kg i.v.) administered 5 min prior to the pyrogen. 5. Centrally administered CRF-41 receptor antagonist (2.5 micrograms/kg I.C.V.) failed to affect the febrile response to poly(I).poly(C) (2.5 micrograms/kg i.v.). 6. CRF-41 immunoneutralization after the onset of temperature rises caused an immediate and significant defervescence. 7. In conclusion, these results suggest a modulatory pro-pyretic role for endogenous peripheral CRF-41 in the febrile responses to poly(I).poly(C).

Central and peripheral prostaglandins are involved in sickness behavior in birds

Many of the behavioral manifestations of mammals and birds following infection are now recognized as important mechanisms for maintaining homeostasis and promoting recovery. To investigate the role of prostaglandins (PGs) in the behavioral and physiological effects of lipopolysaccharide (LPS) in birds, chickens were injected with indomethacin (Ind) peripherally (IP, 5 mg) or centrally (ICV, 100 micrograms) and their behavior and body temperature following a challenge IP injection of LPS (2.5 mg) were assessed at 1 and 2 h, respectively. Pretreatment with Ind IP or ICV completely inhibited the hyperthermia caused by LPS. Ind injected IP but not ICV significantly attenuated the LPS-induced anorexia. The drowsiness caused by LPS was completely inhibited by Ind injected IP and partially inhibited by Ind administered ICV. These results are interpreted to indicate that LPS induces hyperthermia in the chicken by activating a PG system in the brain. Peripheral PGs appear to be involved in the anorectic response to LPS, whereas drowsiness caused by LPS may involve both peripheral and central PGs. These data are consistent with the hypothesis that multiple PG systems are activated during the acute-phase response, which may explain the dissociation between mechanisms controlling the behavioral and physiological responses to infection.

Does an increase in prostaglandin E2 in the blood circulation contribute to a febrile response in rabbits?

We investigated the effect of intravenous injection of human recombinant interleukin-1 beta (IL-1) on rectal temperature and prostaglandin E2 concentration in venous and arterial blood and in the push-pull perfusate in the third ventricle of rabbits. Changes in plasma prostaglandin E2 concentration in blood obtained from the marginal ear vein paralleled changes in body temperature during both monophasic and biphasic fevers. The plasma concentration of prostaglandin E2 in blood obtained from the jugular vein increased during the first phase of the biphasic fever. However, no increase in the prostaglandin E2 level in the carotid arterial blood was observed during the biphasic fever. The levels of prostaglandin E2 in the push-pull perfusate in the third ventricle were markedly elevated during both monophasic and biphasic fevers. Intracarotid infusion of prostaglandin E2 did not produce a fever nor result in a change in the prostaglandin E2 concentration of the push-pull perfusate in the third ventricle. The present results suggest that prostaglandin E2 from the blood circulation does not contribute to fever production in rabbits.

Fatal hyperthermia in a quadriplegic man. Possible evidence for a peripheral action of haloperidol in neuroleptic malignant syndrome

A patient with a cervical cord transection isolating his hypothalamic thermoregulatory centers from peripheral effectors suffered a fatal hyperthermic episode after receiving haloperidol. This suggests that neuroleptic malignant syndrome is caused by a peripheral, not central, effect of haloperidol.

Mechanisms of early and late hypermetabolism and fever after localized tissue injury in rats

Inflammation and localized tissue injury were induced in rats by intramuscular injection of turpentine. This caused rapid and sustained (2-24 h) increases in colonic temperature (Tc; up by 2.5 degrees C) and oxygen consumption (VO2; up by 30%) as well as hypophagia (57%). The early responses (0-2 h) were attenuated by anesthesia, C-fiber deafferentation, peripheral but not central injection of a cyclooxygenase inhibitor, or central injection of a neutralizing antibody to corticotropin-releasing factor (CRF). In contrast, the later (18-20 h) changes in Tc and VO2 were unaffected by anesthesia, C-fiber deafferentation, or central injection of a CRF antibody but were inhibited by either peripheral or central administration of a cyclooxygenase inhibitor. beta-Adrenoceptor blockade reduced both phases of the response, and brown adipose tissue activity was significantly elevated from 4 to 24 h after turpentine injection. These data suggest that different mechanisms mediate the metabolic response to localized tissue injury, the early phase involving neural mechanisms characteristic of stress and inflammation, whereas the later phase appears to depend on humoral factors that are commonly associated with infection.

Prevention of endotoxin-induced increase of cerebral oxygen consumption in dogs by propranolol pretreatment

Cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2) were studied in experimental endotoxic shock in dogs. Eight animals were pretreated with a beta-adrenoceptor blocking agent, propranolol (PPL), per os 12 mg/kg once a day for 7 days. Ten animals served as controls. After an intravenous injection of endotoxin, 1 mg/kg, CBF decreased in both groups, with no significant differences between the groups. CMRO2 increased in the control animals by about 18% from the baseline value both 1 and 2 h after the injection of endotoxin. CMRO2 in the PPL-pretreated animals was unchanged after endotoxin. The CMRO2-reactions to endotoxin in control and PPL animals were significantly different after both 1 and 2 h (P less than 0.05). The present results indicate that the increase in CMRO2 following intravenous endotoxin is mediated via beta-adrenoceptors.

Prostaglandin E2 is involved in adrenocorticotrophic hormone release during swimming exercise in rats

1. We found that adrenocorticotrophic hormone (ACTH) release in rats induced by acute swimming exercise or by an intravenous injection of human recombinant interleukin-1 beta (IL-1 beta) was significantly attenuated after chronic exercise. 2. Since involvement of prostaglandins in the ACTH response induced by IL-1 is well known, we investigated the effect of indomethacin, an inhibitor of prostaglandin synthesis, on the ACTH response induced in rats by acute swimming exercise. Pretreatment with an intravenous injection of indomethacin significantly suppressed the ACTH response induced by exercise. The effect of indomethacin (1 and 10 mg/kg) on the ACTH response was dose-dependent. 3. The effect of chronic exercise on the exercise-induced changes in the plasma concentration of prostaglandin E2 was investigated. The plasma concentration of prostaglandin E2 significantly increased after acute exercise in both the control and the chronically exercised rats. However, the increase in the plasma level of prostaglandin E2 was significantly smaller in the chronically exercised group than in the control group. 4. Intravenous injections of prostaglandin E2 produced dose-dependent increases in the plasma concentration of ACTH in rats. 5. The present results suggest that an increase in prostaglandin E2 levels in plasma is involved in the development of the ACTH response induced by exercise.

Lack of pyrogenic tolerance transmission between brain and periphery in the rabbit

Successive injections of lipopolysaccharide (LPS) either intravenously (i.v.) or intracerebroventricularly (i.c.v.) induced pyrogenic tolerance to LPS in rabbits. Tolerance was shown by a decrease of the magnitude of the fever response to repeated doses of LPS, irrespective of the route of pyrogen administration. A significantly greater and more dramatic decrease of the fever index, however, was observed in rabbits made tolerant to pyrogen given i.v. than when the pyrogen was given i.c.v. Transmission of the pyrogenic tolerance between brain and peripheral tissues, however, has not been ascertained.

Functional and structural differences in febrile mechanism between rabbits and rats

1. Febrile responsiveness of rabbits and rats to intravenous (I.V.) or intracerebroventricular injection of human recombinant interleukin-1 alpha (IL-1 alpha), human recombinant interleukin-1 beta (IL-1 beta) and prostaglandin E2 was examined. 2. The I.V. injection of both IL-1 alpha and IL-1 beta produced dose-dependent fever over a range of 0.05-2.0 micrograms/kg. A small dose of IL-1 alpha (0.5 micrograms/kg) or IL-1 beta (0.5 micrograms/kg) produced a monophasic patterned fever in both rabbits and rats. A large dose (2.0 micrograms/kg) of IL-1 alpha or IL-1 beta produced a biphasic fever in rabbits, but monophasic fever in rats. Febrile responses in rabbits induced by I.V. injection of IL-1 alpha or IL-1 beta were significantly greater than those in rats induced by these same injections. Furthermore, in both species, the pyrogenicity of I.V. IL-1 beta was greater than that of IL-1 alpha. 3. The intracerebroventricular injection of both IL-1 alpha and IL-1 beta produced dose-dependent fever over a range of 0.2-20 ng. In rabbits and rats, the ventricular injections of IL-1 alpha and IL-1 beta produced fever with almost the same pattern--monophasic, regardless of injection doses. Although febrile responsiveness of rabbits to ventricular injection of IL-1 alpha was greater than that of rats, responsiveness to IL-1 beta was almost the same in both species. Pyrogenicity of ventricular IL-1 beta was greater than that of IL-1 alpha. However, febrile responses in rats induced by ventricular injections of several doses (2-2000 ng) of prostaglandin E2 were greater than those in rabbits. 4. The present results show that febrile responsiveness of rabbits to I.V. IL-1 is significantly greater than that of rats. However, fever sensitivity within the central nervous system (CNS) of rats is not lower compared with that of rabbits. Therefore, we considered that between the two species there exist structural differences in the organum vasculosum laminae terminalis (OVLT), which is currently believed to be the pathway of pyrogen to the CNS and/or the site of production of pyrogenic prostaglandins. 5. Histological examinations showed that the rabbit's OVLT has two vascular components with capillaries, one in the layer near the third ventricle and the other in the layer near the subarachnoideal space. However, the rat's OVLT only has a single component, in the layer near the subarachnoideal space.(ABSTRACT TRUNCATED AT 400 WORDS)

Hypothalamic neuronal responses to cytokines

Fever has been extensively studied in the past few decades. The hypothesis that hypothalamic thermosensitive neurons play a major role in both normal thermoregulation and in fever production and lysis has particularly helped to advance our understanding of the neuronal mechanisms underlying the response to pyrogens. Furthermore, new data in the study of host defense responses induced by pyrogenic cytokines such as interleukin 1, interferon alpha 2, tumor necrosis factor alpha, and interleukin 6 have demonstrated that those factors have multiple, yet coordinated, regulatory activities in the central nervous system, so that our understanding of the role of the brain in the activity of these agents requires a new perspective and dimension. Thus, recent evidence from our laboratory indicates that blood-borne cytokines may be detected in the organum vasculosum laminae terminalis and transduced there into neuronal signals. Such signals may then affect distinct, but partially overlapping, sets of neuronal systems in the preoptic area of the anterior hypothalamus, mediating directly and/or indirectly the array of various host defense responses characteristic of infection that are thought to be induced by blood-borne cytokines.

Pyrogenic immunomodulators increase the level of prostaglandin E2 in the blood simultaneously with the onset of fever

Blood prostaglandin E2 (PGE2) levels, estimated by radioimmunoassay, and body temperatures of conscious rabbits were measured simultaneously during fever in response to polyinosinic: polycytidylic acid, lipopolysaccharide and interleukin 1/endogenous pyrogen. The effects of the antipyretic agent ketoprofen on both parameters was also studied. Significant rises (in the order of 6- to 8-fold) in the PGE2 level were observed after injection of either of the three pyrogens and occurred simultaneously with the rise in temperature. Ketoprofen given after the onset of fever in response to the pyrogens produced an immediate defervescence and a simultaneous decrease in plasma PGE2. Ketoprofen given before the pyrogens prevented any rise in either body temperature or plasma PGE2 level. When animals were subjected to an environmental temperature of 34 degrees C a hyperthermia was observed without any change in the blood PGE2 level. These results suggest that an increase in the blood PGE2 level may contribute to the pathogenesis of fever.

Ventromedial hypothalamus is highly sensitive to prostaglandin E2 for producing fever in rabbits

1. The febrile responses induced by intraventricular or intrapreoptic (bilateral) injections of prostaglandin E2 (PGE2) were investigated in the same group of rabbits. Both injections produced dose-dependent fever over a range of 100-2000 ng. However the magnitude of febrile responses induced by ventricular injections was significantly greater than those by intrapreoptic injections. This indicates that there exist regions more sensitive to PGE2 than the preoptic region for producing fever. 2. To explore the regions sensitive to PGE2, the effects of microinjection (1 microliter) of PGE2 (50 and 100 ng) on the rectal temperature were extensively examined in the forty regions of the brain stem. The results showed that the preoptic and anterior hypothalamic region, and the ventromedial hypothalamic region are highly sensitive to PGE2 for producing fever. 3. The febrile responses to PGE2 (50-1000 ng) microinjected into the preoptic region were compared with those induced by injection in the ventromedial hypothalamic region. Fever induced by injection in the ventromedial hypothalamic region was significantly greater than that by injection into the preoptic region. 4. Fever induced by PGE2 injected into the ventromedial hypothalamic region was due to increased heat production in the cold environment (10 degrees C), while in 24 degrees C environment heat losses were reduced without significant changes in heat production. 5. The present results show that the ventromedial hypothalamic region is the most sensitive region to PGE2 for producing fever.