Essential role for endothelin ET(B) receptors in fever induced by LPS (E. coli) in rats

Sexual dimorphism of hypothalamic serotonin release during systemic inflammation: Role of endothelin-1

The hypothalamus receives serotonergic projections from the raphe nucleus in a sex-specific manner. During systemic inflammation, hypothalamic levels of serotonin (5-hydroxytryptamine [5-HT]) decrease in male rats. The present study evaluated the involvement of endothelin-1 (ET-1) in the febrile response, hypolocomotion, and changes in hypothalamic 5-HT levels during systemic inflammation in male and female rats. An intraperitoneal injection of lipopolysaccharide (LPS) induced a febrile response and hypolocomotion in both male and female rats. However, although LPS reduced hypothalamic levels of 5-HT and its metabolite 5-hydroxyindol acetic acid (5-HIAA) in male rats, it increased these levels in female rats. An intracerebroventricular injection of the endothelin-B receptor antagonist BQ788 significantly reduced LPS-induced fever and hypolocomotion and changes in hypothalamic 5-HT and 5-HIAA levels in both male and female rats. The i.c.v. administration of ET-1 induced a significant fever and hypolocomotion, but reduced the hypothalamic levels of 5-HT and 5-HIAA in both males and females. These results suggest an important sexual dimorphism during systemic inflammation regarding the release of 5-HT in the hypothalamus. Moreover, ET-1 arises as an important mediator involved in the changes in hypothalamic 5-HT levels in both male and female rats.

ETA receptors are involved in the febrile response induced by high dose of bacterial endotoxin

Previous studies have demonstrated that endothelin-1 (ET-1) is involved in the febrile response induced by lipopolysaccharide (LPS) in male and female rats. This peptide induces fever acting on ET_B receptors in the central nervous system. However, during sepsis, endothelinergic ET_A receptors in the brain also exert an important role reducing the mortality of the animals. The present study evaluated the participation of ET_A receptors in the febrile response induced by different doses LPS in rats. Male Wistar rats were treated with the ET_A receptor antagonist BQ123 before or after the injection of a low dose (10 μg/kg) or a high dose (200 μg/kg) of LPS intraperitoneally. The febrile response was evaluated. The treatment with BQ123, in both protocols did not change the febrile response induced by the lower dose of LPS. The pre-treatment with BQ123 also did not significantly change the febrile response induced by a higher dose of LPS but the post-treatment with the antagonist abolished the febrile response induced by this dose of LPS. These results suggest that even though ET_A receptors are not recruited in the febrile response induced by lower doses of LPS, they are involved in the febrile response induced by high doses of this stimulus.

Role of central endothelin-1 in hyperalgesia, anhedonia, and hypolocomotion induced by endotoxin in male rats

Sickness syndrome is an adaptive response that can be distinguished by specific signs and symptoms, such as fever and generalized hyperalgesia. Endothelin-1 (ET-1) is produced by inflammatory stimuli, including lipopolysaccharide, and involved in the pathogenesis of inflammation and pain by acting through ET_A and ET_B receptors. ET-1 also induces fever by acting on the central nervous system. The present study investigated the role of ET-1 in sickness syndrome responses, including hyperalgesia, anhedonia, and hypolocomotion. Intracerebroventricular ET-1 administration induced mechanical and thermal hyperalgesia in rats, which was ameliorated by the ET_A receptor antagonist BQ123 and exacerbated by the ET_B receptor antagonist BQ788. A cyclooxygenase blocker did not alter hyperalgesia that was induced by ET-1. Lipopolysaccharide administration induced hyperalgesia, and both BQ123 and BQ788 abolished this mechanical hyperalgesia, but the thermal response was only partially blocked. The blockade of ET_A receptors in the hypothalamus also abolished lipopolysaccharide-induced mechanical hyperalgesia, and the ET_B receptor antagonist did not influence this response. Lipopolysaccharide also induced anhedonia, reflected by lower sucrose preference, and reduced locomotor activity. Both antagonists restored locomotor activity, but only BQ788 reversed the reduction of sucrose preference. These results indicate that ET-1 and both ET_A and ET_B receptors are involved in various responses that are related to sickness syndrome, including hyperalgesia, anhedonia, and hypolocomotion, that is induced by LPS. Hypothalamic ET_A but not ET_B receptors are involved in mechanical hyperalgesia that is observed during lipopolysaccharide-induced sickness syndrome.

Fever Induced by Zymosan A and Polyinosinic-Polycytidylic Acid in Female Rats: Influence of Sex Hormones and the Participation of Endothelin-1

Sex differences in the immune response can also affect the febrile response, particularly the fever induced by lipopolysaccharide (LPS). However, other pathogen-associated molecular patterns, such as zymosan A (Zym) and polyinosinic-polycytidylic acid (Poly I:C), also induce fever in male rats with a different time course of cytokine release and different mediators such as endothelin-1 (ET-1). This study investigated whether female sex hormones affect Zym- and Poly I:C-induced fever and the involvement of ET-1 in this response. The fever that was induced by Zym and Poly I:C was higher in ovariectomized (OVX) female rats compared with sham-operated female rats. Estrogen replacement in OVX females reduced Zym- and Poly I:C-induced fever. The ET_B receptor antagonist BQ788 reversed the LPS-induced fever in cycling females but not in OVX females. BQ788 did not alter the fever that was induced by Zym or Poly I:C in either cycling or OVX females. These findings suggest that the febrile response in cycling females is lower, independently of the stimulus that is inducing it and is probably controlled by estrogen. Also, ET-1 seems to participate in the febrile response that was induced by LPS in males and cycling females but not in the LPS-induced fever in OVX females. Additionally, ET-1 was not involved in the febrile response that was induced by Zym or Poly I:C in females.

Immune-mediated febrile response in female rats: Role of central hypothalamic mediators

Lipopolysaccharide (LPS) induces fever through cytokines like receptor-activator of nuclear factor κB ligand (RANKL), triggering mediators like prostaglandins (PG), endothelin-1 (ET-1), corticotrophin-releasing factor (CRF), substance P (SP) and endogenous opioids. LPS-induced fever is reduced in females compared with males except in ovariectomized (OVX) females which show increased fever mediated by PG. The present study aimed to identify the mediators involved in fever in intact and OVX female rats. Fever was induced with LPS (50 μg/kg) intraperitoneally or CRF (2.5 μg), ET-1 (1 pg), morphine (10 μg) and SP (500 ng) intracerebroventricularly in sham-operated and OVX rats. The role of RANKL was evaluated with osteoprotegerin (OPG, 1 μg, intracerebroventricularly). Expression of RANK, CRFI/II, ETB, μ-opioid (MOR) and NK1 receptors was evaluated by confocal microscopy. Besides LPS, only morphine induced fever in OVX rats while all mediators induced fever in sham-operated animals. OPG abolished LPS-induced fever in OVX but not sham-operated animals. Overall, fever involves similar central mediators in cycling females and males but only morphine induced fever in OVX females. Importantly, RANK/RANKL participates in LPS-induced fever in OVX females, as in males but not in cycling females.

Role of CINC-1 and CXCR2 receptors on LPS-induced fever in rats

The classic model of fever induction is based on the administration of lipopolysaccharide (LPS) from Gram-negative bacteria in experimental animals. LPS-induced fever results in the synthesis/release of many mediators that assemble an LPS-fever cascade. We have previously demonstrated that cytokine-induced neutrophil chemoattractant (CINC)-1, a Glu-Leu-Arg (ELR) + chemokine, centrally administered to rats, induces fever and increases prostaglandin E₂ in the cerebrospinal fluid. We now attempt to investigate the involvement of CINC-1 and its functional receptor CXCR2 on the fever induced by exogenous and endogenous pyrogens in rats. We also investigated the effect of reparixin, an allosteric inhibitor of CXCR1/CXCR2 receptors, on fever induced by either systemic administration of LPS or intracerebroventricular injection of CINC-1, as well as TNF-α, IL-1β, IL-6, or ET-1, known mediators of febrile response. Our results show increased CINC-1 mRNA expression in the liver, hypothalamus, CSF, and plasma following LPS injection. Moreover, reparixin administered right before CINC-1 or LPS abolished the fever induced by CINC-1 and significantly reduced the response induced by LPS. In spite of these results, reparixin does not modify the fever induced by IL-1β, TNF-α, and IL-6, but significantly reduces ET-1-induced fever. Therefore, it is plausible to suggest that CINC-1 might contribute to LPS-induced fever in rats by activating CXCR2 receptor on the CNS. Moreover, it can be hypothesized that CINC-1 is placed upstream TNF-α, IL-1β, and IL-6 among the prostaglandin-dependent fever-mediator cascade and amidst the prostaglandin-independent synthesis pathway of fever.

Central mediators of the zymosan-induced febrile response

BACKGROUND

Zymosan is a fungal cell wall protein-carbohydrate complex that is known to activate inflammatory pathways through the Toll-like receptors and is commonly used to induce fever. Nevertheless, the central mediators that are involved in the zymosan-induced febrile response are only partially known.

METHODS

The present study evaluated the participation of prostaglandins, substance P, endothelin-1 (ET-1), and endogenous opioids (eOPs) in the zymosan-induced febrile response by using inhibitors and antagonists in male Wistar rats.

RESULTS

Both nonselective (indomethacin) and selective (celecoxib) cyclooxygenase inhibitors reduced the febrile response induced by an intraperitoneal (i.p.) injection of zymosan. Indomethacin also blocked the increase in the prostaglandin E2 levels in the cerebrospinal fluid. An intracerebroventricular injection of the neurokinin-1, ETB, and μ-opioid receptor antagonists also reduced the febrile response induced by the i.p. injected zymosan. Moreover, the μ-opioid receptor antagonist CTAP also reduced the febrile response induced by intra-articular injection of zymosan.

CONCLUSIONS

These results demonstrate that prostaglandins, substance P, ET-1, and eOPs are central mediators of the zymosan-induced febrile response.

Female Sex Hormones Influence the Febrile Response Induced by Lipopolysaccharide, Cytokines and Prostaglandins but not by Interleukin-1β in Rats

There are differences in the immune response, and particularly fever, between males and females. In the present study, we investigated how the febrile responses induced by lipopolysaccharide (LPS) and different endogenous pyrogens were affected by female gonadal hormones. The febrile response to i.p. injection of LPS (50 μg/kg) was 40% lower in female rats compared to male or ovariectomised (OVX) female rats. Accordingly, oestrogen replacement in OVX animals reduced LPS-induced fever. Treatment with the prostaglandin synthesis inhibitor indomethacin (2 mg/kg, i.p. 30 min before) reduced the febrile response induced by LPS in both OVX (88%) and sham-operated (71%) rats. In line with the enhanced fever in OVX rats, there was increased expression of cyclooxygenase-2 (COX-2) in the hypothalamus and elevated levels of prostaglandin E₂ (PGE₂ ). In addition, OVX rats were hyper-responsive to PGE₂ injected i.c.v. By contrast to the enhanced fever in response to LPS and PGE₂ , the febrile response induced by i.c.v. injection of interleukin (IL)-1β was unaffected by ovariectomy, whereas the responses induced by tumour necrosis factor (TNF)-α and macrophage inflammatory protein (MIP)-1α were completely abrogated. These results suggest that the mediators involved in the febrile response in females are similar to males, although the reduction of female hormones may decrease the responsiveness of some mediators such as TNF-α and MIP-1α. Compensatory mechanisms may be activated in females after ovariectomy such as an augmented synthesis of COX-2 and PGE₂ .

Central mediators involved in the febrile response: effects of antipyretic drugs

Fever is a complex signal of inflammatory and infectious diseases. It is generally initiated when peripherally produced endogenous pyrogens reach areas that surround the hypothalamus. These peripheral endogenous pyrogens are cytokines that are produced by leukocytes and other cells, the most known of which are interleukin-1β, tumor necrosis factor-α, and interleukin-6. Because of the capacity of these molecules to induce their own synthesis and the synthesis of other cytokines, they can also be synthesized in the central nervous system. However, these pyrogens are not the final mediators of the febrile response. These cytokines can induce the synthesis of cyclooxygenase-2, which produces prostaglandins. These prostanoids alter hypothalamic temperature control, leading to an increase in heat production, the conservation of heat, and ultimately fever. The effect of antipyretics is based on blocking prostaglandin synthesis. In this review, we discuss recent data on the importance of prostaglandins in the febrile response, and we show that some endogenous mediators can still induce the febrile response even when known antipyretics reduce the levels of prostaglandins in the central nervous system. These studies suggest that centrally produced mediators other than prostaglandins participate in the genesis of fever. Among the most studied central mediators of fever are corticotropin-releasing factor, endothelins, chemokines, endogenous opioids, and substance P, which are discussed herein. Additionally, recent evidence suggests that these different pathways of fever induction may be activated during different pathological conditions.

Central mediators involved in the febrile response: effects of antipyretic drugs

Fever is a complex signal of inflammatory and infectious diseases. It is generally initiated when peripherally produced endogenous pyrogens reach areas that surround the hypothalamus. These peripheral endogenous pyrogens are cytokines that are produced by leukocytes and other cells, the most known of which are interleukin-1β, tumor necrosis factor-α, and interleukin-6. Because of the capacity of these molecules to induce their own synthesis and the synthesis of other cytokines, they can also be synthesized in the central nervous system. However, these pyrogens are not the final mediators of the febrile response. These cytokines can induce the synthesis of cyclooxygenase-2, which produces prostaglandins. These prostanoids alter hypothalamic temperature control, leading to an increase in heat production, the conservation of heat, and ultimately fever. The effect of antipyretics is based on blocking prostaglandin synthesis. In this review, we discuss recent data on the importance of prostaglandins in the febrile response, and we show that some endogenous mediators can still induce the febrile response even when known antipyretics reduce the levels of prostaglandins in the central nervous system. These studies suggest that centrally produced mediators other than prostaglandins participate in the genesis of fever. Among the most studied central mediators of fever are corticotropin-releasing factor, endothelins, chemokines, endogenous opioids, and substance P, which are discussed herein. Additionally, recent evidence suggests that these different pathways of fever induction may be activated during different pathological conditions.

Cytokines, but not corticotropin-releasing factor and endothelin-1, participate centrally in the febrile response in zymosan-induced arthritis in rats

Recent literature has revealed that centrally generated prostaglandins participate in the febrile response in zymosan-induced arthritis in rats. However, it is not clear whether other centrally acting pyrogenic mediators such as cytokines, endothelins (ETs), and the corticotropin-releasing factor (CRF) contribute to the febrile response in this model. In the present study, rats were pretreated with intracerebroventricular (i.c.v.) injections of soluble TNF receptor I (sTNFRI), recombinant IL-1 receptor antagonist (IL-1ra), anti-rat IL-6 monoclonal antibody (AbIL-6), α-helical CRF9-41 (a nonselective CRF1/CRF2 receptor antagonist), BQ-123 (an ETA receptor antagonist), BQ-788 (an ETB receptor antagonist), and artificial cerebrospinal fluid (aCSF, control) prior to an intra-articular zymosan (4 mg) injection. Rectal temperatures were measured with a telethermometer. The administration of IL-1ra (200 µg), sTNFRI (500 ng), and AbIL-6 (5 µg) attenuated body temperature elevations after a zymosan injection. The administration of BQ-788 (3 pmol), BQ-123 (3 pmol), and α-helical CRF9-41 (25 µg) did not affect the zymosan-induced febrile response. All the compounds used to pretreat the animals did not significantly alter their basal body temperatures. Together, the results here demonstrate that the febrile response in zymosan-induced arthritis in rats depends on the centrally acting pyrogenic cytokines TNF-α, IL-1β, and IL-6, but does not depend on either CRF or ET-1.

Central mediators involved in the febrile response induced by polyinosinic-polycytidylic acid: lack of involvement of endothelins and substance P

The present study evaluated the involvement of interleukin(IL)-1β, tumor necrosis factor-α (TNF-α), IL-6, interferon(IFN)-γ, prostaglandins of the E2 series, endothelins, substance P and opioids within the central nervous system in polyinosinic:polycytidylic acid (Poly I:C)-induced fever in rats. Poly I:C injection induced a febrile response which was reduced by intracerebroventricular administration of the antibodies against TNF-α, IL-6, or IFN-γ, or by IL-1 or μ receptor antagonists. Intraperitoneal injection of indomethacin or oral administration of celecoxib also reduced Poly I:C-induced fever. Poly I:C increased prostaglandin E2 levels in the cerebrospinal fluid of the animals which was also reduced by indomethacin. The intracerebroventricular injection of ETB or NK1 receptor antagonists did not alter Poly I:C-induced fever. These data suggest the involvement of IL-1β, TNF-α, IL-6, IFN-γ, prostaglandin E2, and opioids but not endothelins and substance P on Poly I:C-induced fever.

The endothelin system has a significant role in the pathogenesis and progression of Mycobacterium tuberculosis infection

Tuberculosis (TB) remains a major global health problem, and although multiple studies have addressed the relationship between Mycobacterium tuberculosis and the host on an immunological level, few studies have addressed the impact of host physiological responses. Proteases produced by bacteria have been associated with important alterations in the host tissues, and a limited number of these enzymes have been characterized in mycobacterial species. M. tuberculosis produces a protease called Zmp1, which appears to be associated with virulence and has a putative action as an endothelin-converting enzyme. Endothelins are a family of vasoactive peptides, of which 3 distinct isoforms exist, and endothelin 1 (ET-1) is the most abundant and the best-characterized isoform. The aim of this work was to characterize the Zmp1 protease and evaluate its role in pathogenicity. Here, we have shown that M. tuberculosis produces and secretes an enzyme with ET-1 cleavage activity. These data demonstrate a possible role of Zmp1 for mycobacterium-host interactions and highlights its potential as a drug target. Moreover, the results suggest that endothelin pathways have a role in the pathogenesis of M. tuberculosis infections, and ETA or ETB receptor signaling can modulate the host response to the infection. We hypothesize that a balance between Zmp1 control of ET-1 levels and ETA/ETB signaling can allow M. tuberculosis adaptation and survival in the lung tissues.

Involvement of brain cytokines in zymosan-induced febrile response

This study compared the involvement of interleukin-1β (IL-1β), IL-6, and tumor necrosis factor-α (TNF-α) within the central nervous system (CNS) in the febrile response induced by zymosan (zym) and lipopolysaccharide (LPS). In addition, we investigated whether zym could activate important regions related to fever; namely, the vascular organ of the laminae terminalis (OVLT) and the median preoptic nucleus (MnPO). Intraperitoneal injection of zym (1, 3, and 10 mg/kg) induced a dose-related increase in core temperature. Zym (3 mg/kg) also reduced tail skin temperature, suggesting the activation of heat conservation mechanisms, as expected, during fever. LPS increased plasma levels of TNF-α measured at 1 h, IL-1β measured at 2 h, and IL-6 measured at 3 h after injection. Zym increased circulating levels of IL-6 but not those of TNF-α or IL-1β at the same time points. In addition, an intracerebroventricular injection of antibodies against TNF-α (2.5 μg) and IL-6 (10 μg) or the IL-1 receptor antagonist (160 ng) reduced the febrile response induced by zym and LPS. Zym (100 μg/ml) also increased intracellular calcium concentration in the OVLT and MnPO from rat primary neuroglial cultures and increased release of TNF-α and IL-6 into the supernatants of these cultures. Together, these results suggest that TNF-α, IL-1β, and IL-6 within the CNS participate in the febrile response induced by zym. However, the time course of release of these cytokines may be different from that of LPS. In addition, zym can directly activate the brain areas related to fever.

Endothelin-1 and its role in the pathogenesis of infectious diseases

Endothelins are potent regulators of vascular tone, which also have mitogenic, apoptotic, and immunomodulatory properties (Rubanyi and Polokoff, 1994; Kedzierski and Yanagisawa, 2001; Bagnato et al., 2011). Three isoforms of endothelin have been identified to date, with endothelin-1 (ET-1) being the best studied. ET-1 is classically considered a potent vasoconstrictor. However, in addition to the effects of ET-1 on vascular smooth muscle cells, the peptide is increasingly recognized as a pro-inflammatory cytokine (Teder and Noble, 2000; Sessa et al., 1991). ET-1 causes platelet aggregation and plays a role in the increased expression of leukocyte adhesion molecules, the synthesis of inflammatory mediators contributing to vascular dysfunction. High levels of ET-1 are found in alveolar macrophages, leukocytes (Sessa et al., 1991) and fibroblasts (Gu et al., 1991). Clinical and experimental data indicate that ET-1 is involved in the pathogenesis of sepsis (Tschaikowsky et al., 2000; Goto et al., 2012), viral and bacterial pneumonia (Schuetz et al., 2008; Samransamruajkit et al., 2002), Rickettsia conorii infections (Davi et al., 1995), Chagas disease (Petkova et al., 2000, 2001), and severe malaria (Dai et al., 2012; Machado et al., 2006; Wenisch et al., 1996a; Dietmann et al., 2008). In this minireview, we will discuss the role of endothelin in the pathogenesis of infectious processes.

The antipyretic effect of dipyrone is unrelated to inhibition of PGE(2) synthesis in the hypothalamus

BACKGROUND AND PURPOSE

Bacterial lipopolysaccharide (LPS) induces fever through two parallel pathways; one, prostaglandin (PG)-dependent and the other, PG-independent and involving endothelin-1 (ET-1). For a better understanding of the mechanisms by which dipyrone exerts antipyresis, we have investigated its effects on fever and changes in PGE(2) content in plasma, CSF and hypothalamus induced by either LPS or ET-1.

EXPERIMENTAL APPROACH

Rats were given (i.p.) dipyrone (120 mg·kg(-1)) or indomethacin (2 mg·kg(-1)) 30 min before injection of LPS (5 µg·kg(-1), i.v.) or ET-1 (1 pmol, i.c.v.). Rectal temperature was measured by tele-thermometry. PGE(2) levels were determined in the plasma, CSF and hypothalamus by elisa.

KEY RESULTS

LPS or ET-1 induced fever and increased CSF and hypothalamic PGE(2) levels. Two hours after LPS, indomethacin reduced CSF and hypothalamic PGE(2) but did not inhibit fever, while at 3 h it reduced all three parameters. Three hours after ET-1, indomethacin inhibited the increase in CSF and hypothalamic PGE(2) levels but did not affect fever. Dipyrone abolished both the fever and the increased CSF PGE(2) levels induced by LPS or ET-1 but did not affect the increased hypothalamic PGE(2) levels. Dipyrone also reduced the increase in the venous plasma PGE(2) concentration induced by LPS.

CONCLUSIONS AND IMPLICATIONS

These findings confirm that PGE(2) does not play a relevant role in ET-1-induced fever. They also demonstrate for the first time that the antipyretic effect of dipyrone was not mechanistically linked to the inhibition of hypothalamic PGE(2) synthesis.

Acute starvation alters lipopolysaccharide-induced fever in leptin-dependent and -independent mechanisms in rats

A decrease in leptin levels with the onset of starvation triggers a myriad of physiological responses including immunosuppression and hypometabolism/hypothermia, both of which can counteract the fever response to pathogens. Here we examined the role of leptin in LPS-induced fever in rats that were fasted for 48 h prior to inflammation with or without leptin replacement (12 μg/day). The preinflammation fasting alone caused a progressive hypothermia that was almost completely reversed by leptin replacement. The LPS (100 μg/kg)-induced elevation in core body temperature ( T core) was attenuated in the fasted animals at 2–6 h after the injection, an effect that was not reversed by leptin replacement. Increasing the LPS dose to 1,000 μg/kg caused a long-lasting fever that remained unabated for up to 36 h after the injection in the fed rats. This sustained response was strongly attenuated in the fasted rats whose T core started to decrease by 18 h after the injection. Leptin replacement almost completely restored the prolonged fever. The attenuation of the prolonged fever in the fasted animals was accompanied by the diminution of proinflammatory PGE2 in the cerebrospinal fluid and mRNA of proopiomelanocortin (POMC) in the hypothalamus. Leptin replacement prevented the fasting-induced reduction of POMC but not PGE2. Moreover, the leptin-dependent fever maintenance correlated closely with hypothalamic POMC levels ( r = 0.77, P < 0.001). These results suggest that reduced leptin levels during starvation attenuate the sustained fever response by lowering hypothalamic POMC tone but not PGE2 synthesis.

Quercetin does not alter lipopolysaccharide-induced fever in rats

Fever is considered an important component of the acute phase response of the body in defence against invading organisms such as bacteria. Quercetin, an important representative of the flavonoid class, has been extensively studied as an anti-inflammatory agent. In the present study, we investigated the effect of quercetin, administered orally (5, 25 and 50 mg kg−1) or intraperitoneally (50 mg kg−1), on the febrile response induced by either intraperitoneally (50 μg kg−1) or intravenously (5 μg kg−1) injected lipopolysaccharide (LPS from Escherichia coli) in rats. In contrast with the well known anti-inflammatory activity of quercetin, the results demonstrate that quercetin, at the doses used, did not alter the fever induced by LPS, regardless of the route of administration.

Endogenous cannabinoids induce fever through the activation of CB1 receptors

BACKGROUND AND PURPOSE

The effects of centrally administered cannabinoids on body core temperature (Tc) and the contribution of endogenous cannabinoids to thermoregulation and fever induced by lipopolysaccharide (LPS) (Sigma Chem. Co., St. Louis, MO, USA) were investigated.

EXPERIMENTAL APPROACH

Drug-induced changes in Tc of male Wistar rats were recorded over 6 h using a thermistor probe (Yellow Springs Instruments 402, Dayton, OH, USA) inserted into the rectum.

KEY RESULTS

Injection of anandamide [(arachidonoylethanolamide (AEA); Tocris, Ellisville, MO, USA], 0.01-1 microg i.c.v. or 0.1-100 ng intra-hypothalamic (i.h.), induced graded increases in Tc (peaks 1.5 and 1.6 degrees C at 4 h after 1 microg i.c.v. or 10 ng i.h.). The effect of AEA (1 microg, i.c.v.) was preceded by decreases in tail skin temperature and heat loss index (values at 1.5 h: vehicle 0.62, AEA 0.48). Bell-shaped curves were obtained for the increase in Tc induced by the fatty acid amide hydrolase inhibitor [3-(3-carbamoylphenyl)phenyl] N-cyclohexylcarbamate (Cayman Chemical Co., Ann Arbor, MI, USA) (0.001-1 ng i.c.v.; peak 1.9 degrees C at 5 h after 0.1 ng) and arachidonyl-2-chloroethylamide (ACEA; Tocris) (selective CB(1) agonist; 0.001-1 microg i.c.v.; peak 1.4 degrees C 5 h after 0.01 microg), but (R,S)-(+)-(2-Iodo-5-nitrobenzoyl)-[1-(1-methyl-piperidin-2-ylmethyl)-1H-indole-3-yl] methanone (Tocris) (selective CB(2) agonist) had no effect on Tc. AEA-induced fever was unaffected by i.c.v. pretreatment with 6-Iodo-2-methyl-1-[2-(4-morpholinyl)ethyl]-1H-indole-3-yl](4-methoxyphenyl) methanone (Tocris) (selective CB(2) antagonist), but reduced by i.c.v. pretreatment with N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251; Tocris) (selective CB(1) antagonist). AM251 also reduced the fever induced by ACEA or LPS.

CONCLUSIONS AND IMPLICATIONS

The endogenous cannabinoid AEA induces an integrated febrile response through activation of CB(1) receptors. Endocannabinoids participate in the development of the febrile response to LPS constituting a target for antipyretic therapy.

Mechanisms operated by endothelin ETA and ETB receptors in the trigeminal ganglion contribute to orofacial thermal hyperalgesia induced by infraorbital nerve constriction in rats

Endothelins, acting through specific endothelin ET(A) and/or ET(B) receptors, participate in nociceptive processing in models of cancer, inflammatory and neuropathic pain. The present study investigated which cell types express endothelin receptors in the trigeminal ganglion, and the contribution of mechanisms mediated by endothelin ET(A) and ET(B) receptors to orofacial heat hyperalgesia induced by unilateral constriction of the infraorbital nerve (CION). Both receptor types were identified by immunohistochemistry in the trigeminal ganglion, ET(A) receptors on small-sized non-myelinated and myelinated A-fibers and ET(B) receptors on both satellite glial cells and small-sized non-myelinated neuronal cells. CION promoted ipsilateral orofacial heat hyperalgesia which lasted from Day 2 until Day 10 after surgery. Ongoing CION-induced heat hyperalgesia (on Day 4) was reduced transiently, but significantly, by systemic or local treatment with antagonists of endothelin ET(A) receptors (atrasentan, 10 mg/kg, i.v.; or BQ-123, 10 nmol/lip), endothelin ET(B) receptors (A-192621, 20 mg/kg, i.v.; or BQ-788, 10 nmol/ lip), or of both ET(A)/ET(B) receptors (bosentan, 10 mg/kg, i.v.; or BQ-123 plus BQ-788, each at 10 nmol/lip). On the other hand, CION-induced heat hyperalgesia was transiently abolished over the first 90 min following i.p. injection of morphine hydrochloride (2.5 mg/kg), but fully resistant to reversal by indomethacin (4 mg/kg, i.p.) or celecoxib (10 mg/kg, i.p.). Thus, heat hyperalgesia induced by CION is maintained, in part, by peripheral signaling mechanisms operated by ET(A) and ET(B) receptors. Endothelin receptors might represent promising therapeutic targets for the control of trigeminal neuropathic pain.

Endogenous opioids: role in prostaglandin-dependent and -independent fever

This study evaluated the participation of μ-opioid-receptor activation in body temperature (Tb) during normal and febrile conditions (including activation of heat conservation mechanisms) and in different pathways of LPS-induced fever. The intracerebroventricular treatment of male Wistar rats with the selective opioid μ-receptor-antagonist cyclic d-Phe-Cys-Try-d-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP; 0.1–1.0 μg) reduced fever induced by LPS (5.0 μg/kg) but did not change Tb at ambient temperatures of either 20°C or 28°C. The subcutaneous, intracerebroventricular, and intrahypothalamic injection of morphine (1.0–10.0 mg/kg, 3.0–30.0 μg, and 1–100 ng, respectively) produced a dose-dependent increase in Tb. Intracerebroventricular morphine also produced a peripheral vasoconstriction. Both effects were abolished by CTAP. CTAP (1.0 μg icv) reduced the fever induced by intracerebroventricular administration of TNF-α (250 ng), IL-6 (300 ng), CRF (2.5 μg), endothelin-1 (1.0 pmol), and macrophage inflammatory protein (500 pg) and the first phase of the fever induced by PGF2α (500.0 ng) but not the fever induced by IL-1β (3.12 ng) or PGE2 (125.0 ng) or the second phase of the fever induced by PGF2α. Morphine-induced fever was not modified by the cyclooxygenase (COX) inhibitor indomethacin (2.0 mg/kg). In addition, morphine injection did not induce the expression of COX-2 in the hypothalamus, and CTAP did not modify PGE2 levels in cerebrospinal fluid or COX-2 expression in the hypothalamus after LPS injection. In conclusion, our results suggest that LPS and endogenous pyrogens (except IL-1β and prostaglandins) recruit the opioid system to cause a μ-receptor-mediated fever.

Mediators involved in the febrile response induced by Tityus serrulatus scorpion venom in rats

Tityus serrulatus venom (Tsv) was intraperitoneally (ip) injected at doses of 75, 150 and 300mug/kg and IL-1beta (2.0 microg/kg) was given intravenously (iv) to male Wistar rats. Rectal temperature was measured by radiotelemetry. Vagotomy was performed according to Bluthe et al. [1994. Lipopolysaccharide induces sickness behaviour in rats by a vagal mediated mechanism. C R Acad. Sci. 317(6), 499-503]. Cerebrospinal fluid (CSF) and peritoneal fluid (PF) levels of bradykinin (BK) were measured by ELISA. B(1) (des-Arg(9)-[Leu(8)]-BK; DALBK) and B(2) kinin receptor (icatibant) antagonists (1.0 mg/kg each), the induced nitric oxide synthase inhibitor aminoguanidine (50.0 mg/kg), the neuronal nitric oxide synthase inhibitor 7-nitroindazole (30.0 mg/kg), the dual cyclooxygenase inhibitor ibuprofen (10.0 mg/kg), the selective interleukin-1 receptor antagonist IL-ra (2.0 mg/kg) and dipyrone (120 mg/kg) were given ip. Celecoxib (5 mg/kg) was given per os (po). Tsv at doses of 75 microg/kg evoked no change in rectal temperature while at doses of 150 and 300 microg/kg it promoted long-lasting fever (2 degrees C+/-0.1). Tsv (150 microg/kg) increased by nearly 3 and 5 times, respectively BK concentration in the CSF and in the PF. Subdiaphragmatic vagotomy or 7-nitroindazole reduced, icatibant, DALBK, IL-1ra, aminoguanidine and dipyrone abolished, while ibuprofen and celecoxib failed to affect Tsv-induced fever. These results suggest that PGs do not play a relevant role, whereas, kinins via their B(1) and B(2) receptors, IL-1, nitric oxide and vagal neurotransmission are involved in Tsv-induced fever.

CCL3/Macrophage inflammatory protein-1α induces fever and increases prostaglandin E2 in cerebrospinal fluid of rats: Effect of antipyretic drugs

The aim of this study was to investigate whether the increase in body temperature caused by intracerebroventricular (i.c.v.) injection of recombinant mouse CCL3/MIP1alpha [C-C (two adjacent conserved cysteines) ligand 3/macrophage inflammatory protein-1alpha] constitutes solely a hyperthermic response or a true integrated fever. Additionally, we examined the effects of systemic administration of different antipyretic drugs including the glucocorticoid dexamethasone, on cerebrospinal fluid (CSF) concentration of prostaglandin (PG) E2 and on febrile response induced by CCL3/MIP1alpha. I.c.v. administration of CCL3/MIP1alpha evokes an integrated fever accompanied by a reduction in tail skin temperature and an increase in PGE2 concentration in the CSF. Dexamethasone and indomethacin markedly reduced the fever and the elevation of CSF PGE2 concentration induced by lipopolysaccharide (LPS) whereas both response evoked by i.c.v. CCL3/MIP1alpha were insensitive to this steroid. Indomethacin only blocked the PGE2 increase in the CSF whereas ibuprofen and celecoxib each blocked the fever and the elevation of CSF PGE2. In this study, we have demonstrated for the first time that CCL3/MIP1alpha evokes an integrated febrile response accompanied by an increase of PGE2 levels in the CSF. These events are dissociated, especially in animals treated with indomethacin. If PGE2 does not participate in the febrile response evoked by CCL3/MIP1alpha, the inhibition of this response by celecoxib and ibuprofen indicates additional mechanisms to the well-known inhibition of COX enzymes by these drugs. Such mechanisms do not seem to depend on cytokine synthesis and subsequent COX-2 induction.

Physiology of temperature regulation: comparative aspects

Few environmental factors have a larger influence on animal energetics than temperature, a fact that makes thermoregulation a very important process for survival. In general, endothermic species, i.e., mammals and birds, maintain a constant body temperature (Tb) in fluctuating environmental temperatures using autonomic and behavioural mechanisms. Most of the knowledge on thermoregulatory physiology has emerged from studies using mammalian species, particularly rats. However, studies with all vertebrate groups are essential for a more complete understanding of the mechanisms involved in the regulation of Tb. Ectothermic vertebrates-fish, amphibians and reptiles-thermoregulate essentially by behavioural mechanisms. With few exceptions, both endotherms and ectotherms develop fever (a regulated increase in Tb) in response to exogenous pyrogens, and regulated hypothermia (anapyrexia) in response to hypoxia. This review focuses on the mechanisms, particularly neuromediators and regions in the central nervous system, involved in thermoregulation in vertebrates, in conditions of euthermia, fever and anapyrexia.

Endothelin-1 as a central mediator of LPS-induced fever in rats

Fever induced by E. coli lipopolysaccharide (LPS) in rats is substantially reduced by blockade of central endothelin ET(B) receptors. This study explores the role of endothelin-1 as a central mediator of fever in rats, by investigating the effect of a pyrogenic dose of LPS on the levels of big endothelin-1 and endothelin-1 in the cerebrospinal fluid (CSF) and endothelin-1 in the plasma. We further assessed whether the increase in body temperature caused by central injection of endothelin-1 constitutes solely a hyperthermia or a true integrated febrile response. LPS (5 mug kg(-1), i.v.) induced fever which peaked at 1.16 +/- 0.24 degrees C within 2 h and remained stable up to 5 h. CSF levels of immunoreactive (ir) big endothelin-1 decreased to undetectable levels at 3 h after LPS, returning only partially at 5 h post-injection. CSF ir-endothelin-1 levels were undetectable in saline-treated animals, but reached 21.9 +/- 5.2 fmol ml(-1) at 3 h after LPS treatment. Plasma ir-endothelin-1 levels were unchanged after saline or LPS. Central injection of endothelin-1 (1 pmol, i.c.v.) caused long-lasting increases in body temperature (0.81 +/- 0.17 degrees C, 3 h), but simultaneously decreased tail skin temperature (-1.10 +/- 0.26 degrees C), indicating cutaneous vasoconstriction. Moreover, endothelin-1 induced fever (1.0 +/- 0.3 degrees C, 3 h) when injected into the preoptic area of the anterior hypothalamus (100 fmol), but not i.v. (1 or 10 pmol). These data suggest that endothelin-1 is produced in the brain and acts centrally as a mediator of LPS-induced fever.

Interleukin-1 mediates endothelin-1-induced fever and prostaglandin production in the preoptic area of rats

The intracerebroventricular injection of endothelin-1 (ET-1) induces fever and increases PG levels in the cerebrospinal fluid of rats. Likewise, the injection of IL-1 into the preoptic area (POA) of the rat hypothalamus causes both fever and increased PG production. In this study, we conducted in vivo and in vitro experiments in the rat to investigate 1) the hypothalamic region involved in ET-1-induced fever and PG biosynthesis and 2) whether hypothalamic IL-1 plays a role as a mediator of the above ET-1 activities. One hundred femtomoles of ET-1 increased body temperature when injected in the POA of conscious Wistar rats; this effect was significantly counteracted by the coinjection of 600 pmol IL-1 receptor antagonist (IL-1ra). In experiments on rat hypothalamic explants, 100 nM ET-1 caused a significant increase in PGE2 production and release from the whole hypothalamus and from the isolated POA, but not from the retrochiasmatic region, in 1-h incubations. Six nanomoles of IL-1ra or 10 nM of a cell-permeable interleukin-1 converting enzyme inhibitor completely counteracted the effect of ET-1 on PGE2 release from the POA. One hundred nanomoles ET-1 also caused a significant increase in IL-1beta immunoreactivity released into the bath solution of hypothalamic explants after 1 h of incubation, although during such time ET-1 failed to modify the gene expression of IL-1beta and other pyrogenic cytokines within the hypothalamus. In conclusion, our results show that ET-1 increases IL-1 production in the POA, and this effect appears to be correlated to ET-1-induced fever in vivo, as well as to PG production in vitro.

Central endothelin ETBreceptors mediate IL-1-dependent fever induced by preformed pyrogenic factor and corticotropin-releasing factor in the rat

Blockade of central endothelin ETBreceptors inhibits fever induced by LPS in conscious rats. The contribution of ETBreceptor-mediated mechanisms to fever triggered by intracerebroventricular IL-6, PGE2, PGF2α, corticotropin-releasing factor (CRF), and preformed pyrogenic factor derived from LPS-stimulated macrophages (PFPF) was examined. The influence of natural IL-1 receptor antagonist or soluble TNF receptor I on endothelin (ET)-1-induced fever was also assessed. The selective ETBreceptor antagonist BQ-788 (3 pmol icv) abolished fever induced by intracerebroventricular ET-1 (1 pmol) or PFPF (200 ng) and reduced that caused by ICV CRF (1 nmol) but not by IL-6 (14.6 pmol), PGE2(1.4 nmol), or PGF2α(2 nmol). CRF-induced fever was also attenuated by bosentan (dual ETA/ETBreceptor antagonist; 10 mg/kg iv) but unaffected by BQ-123 (selective ETAreceptor antagonist; 3 pmol icv). α-Helical CRF9–41(dual CRF1/CRF2receptor antagonist; 6.5 nmol icv) attenuated fever induced by CRF but not by ET-1. Human IL-1 receptor antagonist (9.1 pmol) markedly reduced fever to IL-1β (180 fmol) or ET-1 and attenuated that caused by PFPF or CRF. Murine soluble TNF receptor I (23.8 pmol) reduced fever to TNF-α (14.7 pmol) but not to ET-1. The results of the present study suggest that PFPF and CRF recruit the brain ET system to cause ETBreceptor-mediated IL-1-dependent fever.

Systemic administration of lipopolysaccharide upregulates angiotensin II expression in rat renal tubules: immunohistochemical and ELISA studies

We investigated whether angiotensin II (AII) peptide is induced in the rat kidney under endotoxemic conditions. Immunohistochemistry revealed strong AII-like immunoreactivity in the renal tubules of rats given high-dose lipopolysaccharide (LPS; 1000 microg/kg) intraperitoneally (i.p.). AII-like immunoreactivity in renal tubules was slight at 1h after the LPS injection, but marked at 3 h. There were few signals in the kidney in saline-injected control rats. When injected at 0.1, 10, or 1000 microg/kg i.p., LPS-induced a dose-related increase in AII-like immunoreactivity in renal tubules that was unaffected by treatment with the prostaglandin-synthesis blocker indomethacin. ELISA measurement of the AII concentration in the whole kidney supported the above findings. These results suggest that systemically administered LPS induces AII peptide expression in renal tubules by a prostaglandin-independent mechanism.

Endothelins induce ETB receptor-mediated mechanical hypernociception in rat hindpaw: roles of cAMP and protein kinase C

The present study assesses the capacity of endothelins to induce mechanical hypernociception, and characterises the receptors involved and the contribution of cAMP and protein kinases A (PKA) and C (PKC) to this effect. Intraplantar administration of endothelin-1, endothelin-2 or endothelin-3 (3-30 pmol) induced dose- and time-dependent mechanical hypernociception, which was inhibited by BQ-788 (N-cys-2,6-dimethylpiperidinocarbonyl-l-gamma-methylleucyl-d-1-methoxycarboyl-d-norleucine; endothelin ET(B) receptor antagonist), but not BQ-123 (cyclo[d-Trp-d-Asp-Pro-d-Val-Leu]; endothelin ET(A) receptor antagonist; each at 30 pmol). The selective endothelin ET(B) receptor agonist BQ-3020 (N-Ac-Ala(11,15)-endothelin-1 (6-21)) fully mimicked the hypernociceptive effects of the natural endothelins. Treatments with indomethacin, atenolol or dexamethasone did not inhibit endothelin-1-evoked mechanical hypernociception. However, endothelin-1-induced mechanical hypernociception was potentiated by the cAMP phosphodiesterase inhibitor rolipram (4-[3-(cyclopentyloxy)-4-methoxyphenyl]-2-pyrrolidinone) and inhibited by the PKC inhibitors staurosporine and calphostin C, but was unaffected by the PKA inhibitor H89 (N-[2-((p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide). Thus, endothelins, acting through endothelin ET(B) receptors, induce mechanical hypernociception in the rat hindpaw via cAMP formation and activation of the PKC-dependent phosphorylation cascade.

The effects of selective and nonselective cyclooxygenase inhibitors on endothelin-1-induced fever in rats

It was previously shown that sustained fever can be induced in rats by central injection of endothelin-1 (ET-1). This peptide appears to participate in the mechanism(s) of LPS-induced fever, which is reduced by pretreatments with ETB receptor antagonists. In this study, we compared the effects of a nonselective cyclooxygenase (COX) inhibitor, indomethacin, with those of two selective COX-2 inhibitors, celecoxib and lumiracoxib, on ET-1-induced fever in rats. Fever induced in conscious animals by ET-1 (1 pmol icv) or LPS (5 μg/kg iv) was prevented by pretreatments with celecoxib (5 and 10 mg/kg) or lumiracoxib (5 mg/kg) given by oral gavage 1 h before stimuli. Lower doses of celecoxib had partial (2.5 mg/kg) or no effect (1 mg/kg). Indomethacin (2 mg/kg ip) partially inhibited fever induced by LPS but had no effect on ET-1-induced fever. The levels of PGE2 and PGF2α in the cerebrospinal fluid (CSF) of pentobarbital sodium-anesthetized rats were significantly increased 3 h after the injection of LPS or ET-1. The latter increase was abolished by celecoxib at all tested doses and by indomethacin. In conclusion, selective COX-2 inhibitors were able to prevent ET-1-induced fever, indicating a role for COX-2 in this phenomenon. However, the fact that reduced CSF PG levels obtained with indomethacin and a low dose of celecoxib are not accompanied by changes in fever induced by ET-1, along with the lack of inhibitory effects of indomethacin on ET-1 fever, suggests that the latter might also involve COX-2-independent mechanisms.

Function of the endothelinB receptor in cardiovascular physiology and pathophysiology

One of the two receptors by which the potent vasoactive effects of endothelin (ET)-1 are mediated is the ET(B) receptor (ET(BR)), which is found in several tissues, but, more importantly from a cardiovascular point of view, on the endothelial cell. The endothelial cell also has the unique capability of releasing ET-1, as well as other factors, such as the endothelial-derived relaxing factors and prostacyclin, which counteract the myotropic effects of the peptide. The secretory and contractile responses to ET-1 rely on G-protein-coupled ET(BR)s, as well as ET(A)-G-protein-coupled receptor-like proteins. The mitogenic properties of ET-1 via ET(A) receptors (ET(AR)s) coupled to mitogen-activated protein kinases and tyrosine kinases on the vascular smooth muscle may occur in conjunction with the anti-apoptotic characteristics of the endothelial ET(BR)s. Interestingly, most of the relevant antagonists and agonists for both ET(AR)s and ET(BR)s have been developed by the pharmaceutical industry. This highlights the therapeutical potential of compounds that act on ET receptors. In normal as well as in physiopathological conditions, the ET(BR) plays an important role in the control of vascular tone, and must be taken into account when using ET receptor antagonists for the treatment of cardiovascular diseases. For the management of congestive heart failure, renal failure and primary pulmonary hypertension, the most recent literature supports the use of selective ET(AR) antagonists rather than mixed antagonists of ET(AR)s and ET(BR)s. Nonetheless, validation of this view will have to await the first clinical trials comparing the actions of ET(A) to mixed ET(A)/ET(B) receptor antagonists.

Indomethacin impairs LPS-induced behavioral fever in toads

We tested the hypothesis that PGs mediate lipopolysaccharide (LPS)-induced behavioral fever in the toad Bufo paracnemis. Measurements of preferred body temperature (T(b)) were performed with a thermal gradient. Toads were injected intraperitoneally with the cyclooxygenase inhibitor indomethacin (5 mg/kg), which inhibits PG biosynthesis, or its vehicle (Tris) followed 30 min later by LPS (0.2 and 2 mg/kg) into the lymph sac. LPS at the dose of 0.2 mg/kg caused a significant increase in T(b) from 7 to 10 h after injection, and then T(b) returned toward baseline values. LPS at the dose of 2 mg/kg produced a different pattern of response, with a longer latency to the onset of fever (10th h) and a longer duration (until the end of the experiment at the 15th h). Tris significantly attenuated the fever induced by LPS at 0.2 mg/kg, but not at 2 mg/kg. Moreover, indomethacin completely blocked the fever evoked by LPS (2 mg/kg). These results indicate that the behavioral fever induced by LPS in toads requires the activation of the COX pathway, suggesting that the involvement of PG in fever has an ancient phylogenetic history and that endogenous PGs raise the thermoregulatory set point to produce fever, because behavioral thermoregulation seems to be related to changes in the thermoregulatory set point.

Cyclooxygenase inhibitors attenuate endothelin ET(B) receptor-mediated contraction in human temporal artery

It is well documented that endothelin ET(B) receptor-mediated contraction develops in artery segments incubated in culture and that the reaction is augmented by proinflammatory cytokines, but little is known of the mechanisms involved. Segments of human temporal artery were incubated in organ culture for 2 days in the absence or presence of interleukin-1 beta (IL-1 beta), with or without nonsteroidal anti-inflammatory drugs, glucocorticoids or a nitric oxide synthase inhibitor. Thereafter, contractions were induced by the selective endothelin ET(B) receptor agonist, sarafotoxin S6c. Acetylsalicylic acid, indomethacin, nimesulide and rofecoxib were all effective in eliminating the increase in endothelin ET(B) receptor-mediated contraction induced by interleukin-1 beta, but only indomethacin and rofecoxib significantly reduced the spontaneous development of this reaction in cultured arteries. Dexamethasone and methylprednisolone augmented the reaction, and the nitric oxide synthase inhibitor had no effect. The results clearly indicate a role for cyclooxygenase, most likely cyclooxygenase-2, in endothelin ET(B) receptor-mediated contraction in this preparation.

Effects of heme oxygenase system on the cyclooxygenase in the primary cultured hypothalamic cells

Endogenous carbon monoxide (CO) shares with nitric oxide (NO) a role as a putative neural messenger in the brain. Both gases are believed to modulate CNS function via an increase in cytoplasmic cGMP concentrations secondary to the activation of soluble guanylate cyclase (sGC). Recently CO and NO were proposed as a possible mediator of febrile response in hypothalamus. NO has been reported to activate both the constitutive and inducible isoform of the cyclooxygenase (COX). Thus, we investigated whether CO arising from heme catabolism by heme oxygenase (HO) is involved in the febrile response via the activation of COX in the hypothalamus. PGE2 which is a final mediator of febrile response released from primary cultured hypothalamic cells was taken as a marker of COX activity. PGE2 concentration was measured with EIA kits. Exogenous CO (CO-saturated medium) and hemin (a substrate and potent inducer of HO) evoked an increase in PGE2 release from hypothalamic cells, and these effects were blocked by methylene blue (an inhibitor of sGC). And membrane permeable cGMP analogue, dibutyryl-cGMP elicited significant increases in PGE2 release. These results suggest that there may be a functional link between HO and COX enzymatic activities. The gaseous product of hemin through the HO pathway, CO, might play a role through the modulation of the COX activity in the hypothalamus.

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.

Mechanisms of the cutaneous vasodilator response to local external pressure application in rats: involvement of CGRP, neurokinins, prostaglandins and NO

1. Local pressure-induced vasodilation (PIV) is a neural vasodilator response to non-nociceptive externally applied pressure in the skin, previously described in humans. We first determined whether PIV exists in rats and depends on capsaicin-sensitive fibres as it does in humans. We then examined the mediators involved in the efferent pathway of PIV. 2. Cutaneous blood flow was measured by laser Doppler flowmetry during 11.1 Pa s(-1) increases in local applied pressure in anaesthetized rats. The involvement of capsaicin-sensitive fibres in PIV was tested in rats treated neonatally with capsaicin. To antagonize CGRP, neurokinin-1, -2, or -3 receptors, different groups of rats were treated with CGRP(8 - 37), SR140333, SR48968 or SR142801, respectively. Prostaglandins involvement was tested with indomethacin treatment. To inhibit nitric oxide synthase (NOS) activity or specific neuronal NOS, rats were treated with N(G)-nitro-L-arginine or 7-nitroindazole, respectively. 3. PIV was found in rats, as in humans. PIV was abolished by neonatal treatment with capsaicin and by administration of CGRP(8 - 37) but remained unchanged with SR140333, SR48968 and SR142801 treatments. Prostaglandin inhibition resulted in a significant decrease in PIV. Inhibition of NOS abolished PIV, whereas inhibition of neuronal NOS caused a diminution of PIV. 4. These data suggest that PIV depends on capsaicin-sensitive fibres in rats, as in humans. It appears that CGRP plays a major role in the PIV, whereas neurokinins have no role. Furthermore, PIV involves a contribution from prostaglandins and depends on endothelial NO, whereas neuronal NO has a smaller role.

Central CO-heme oxygenase pathway raises body temperature by a prostaglandin-independent way

Recently, the carbon monoxide (CO)-heme oxygenase pathway has been shown to play an important role in fever generation by acting on the central nervous system, but the mechanisms involved have not been assessed. Thus the present study was designed to determine whether prostagandins participate in the rise in body temperature (T(b)) observed after induction of the CO-heme oxygenase pathway in the central nervous system. Intracerebroventricular (ICV) injection of heme-lysinate (152 nmol/4 microl), which is known to induce the CO-heme oxygenase pathway, caused an increase in T(b) [thermal index (TI) = 5.3 +/- 0.5 degrees C. h], which was attenuated by ICV administration of the heme oxygenase inhibitor ZnDPBG (200 nmol/4 microl; TI = 2.5 +/- 1.7 degrees C. h; P < 0.05). No change in T(b) was observed after intraperitoneal injection of the cyclooxygenase inhibitor indomethacin (5 mg/kg), whereas indomethacin at the same dose attenuated the fever induced by ICV administration of lipopolysaccharide (LPS) (10 ng/2 microl) (vehicle/LPS: TI = 4.5 +/- 0.5 degrees C. h; indomethacin/LPS: TI = 1.7 +/- 1.0 degrees C. h; P < 0.05). Interestingly, indomethacin did not affect the rise in T(b) induced by heme-lysinate (152 nmol/4 microl) ICV injection (vehicle/heme: TI = 4.5 +/- 1.4 degrees C. h; indomethacin/heme: TI = 4.2 +/- 1.0 degrees C. h). Finally, PGE(2) (200 ng/2 microl) injected ICV evoked a rise in T(b) that lasted 1.5 h. The heme oxygenase inhibitor ZnDPBG (200 nmol/4 microl) failed to alter PGE(2)-induced fever. Taken together, these results indicate that the central CO-heme oxygenase pathway increases T(b) independently of prostaglandins.

Endothelin-1-induced ET(A) receptor-mediated nociception, hyperalgesia and oedema in the mouse hind-paw: modulation by simultaneous ET(B) receptor activation

Endothelin-1 causes ET(A) receptor-mediated enhancement of capsaicin-induced nociception in mice. We have assessed if this hyperalgesic effect of endothelin-1 is also accompanied by other pro-inflammatory effects, namely nociception and oedema, and characterized the endothelin ET receptors involved. Intraplantar (i. pl.) hind-paw injection of endothelin-1 (0.3 - 30 pmol) induced graded nociceptive responses (accumulated licking time: vehicle, 20. 5+/-3.3 s; endothelin-1 at 30 pmol, 78.1+/-9.8 s), largely confined to the first 15 min. Endothelin-1 (1 - 10 pmol) potentiated ipsilateral capsaicin-induced (0.1 microgram, i.pl.; at 30 min) nociception (vehicle, 40.2+/-2.6 s; endothelin-1 at 10 pmol, 98.4+/-5.8 s, but 30 pmol was inactive), and caused oedema (increase in paw weight 5 min after capsaicin: vehicle, 46.3+/-2.3 mg; endothelin-1 at 30 pmol, 100.3+/-6.1 mg). Selective ET(B) receptor agonists sarafotoxin S6c (up to 30 pmol) and IRL 1620 (up to 100 pmol) were inactive, whereas endothelin-3 (up to 30 pmol) induced only modest oedema. ET(A) receptor antagonists BQ-123 (1 nmol, i.pl. ) or A-127722-5 (6 micromol kg(-1), i.v.) prevented all effects of endothelin-1 (10 pmol), but the ET(B) receptor antagonist BQ-788 (1 or 10 nmol, i.pl.) was ineffective. BQ-788 (10 nmol, i.pl.) unveiled hyperalgesic effects of 30 pmol endothelin-1 and endothelin-3. Sarafotoxin S6c (30 pmol, i.pl.) did not modify endothelin-1-induced (10 pmol) nociception or oedema, but abolished hyperalgesia. Thus, endothelin-1 triggers ET(A) receptor-mediated nociception, hyperalgesia and oedema in the mouse hind-paw. Simultaneous activation of ET(B) receptors by endothelin-1 or selective agonists can limit the hyperalgesic, but not the nociceptive or oedematogenic, effects of the peptide.

In vivo evidence that activation of tyrosine kinase is a trigger for lipopolysaccharide-induced fever in rats

We measured the rectal temperature of free-moving, conscious rats after intracerebroventricular (i.c.v.) injections of lipopolysaccharide (LPS) and interleukin-1beta (IL-1beta) with or without various antagonists to investigate the mechanisms involved in LPS-induced fever. LPS (3 microg) elicited significant increases in rectal temperature, which lasted from 0.5 h to more than 8 h after administration. This febrile response was inhibited by pretreatment with L-nitro-arginine (LNA), indomethacin (IND), genistein (GEN), tyrphostin 46 and anti-rat IL-1beta antibody (anti-IL-1beta Ab), but was not inhibited by pretreatment with daidzein or chelerythrine (CHE) into the ventricle. LPS (0.3 microg) following orthovanadate (i.c.v.) produced fever, although the small amount of LPS (0.3 microg) or orthovanadate alone showed no effect on rectal temperature. I.c.v. injections of IL-1beta also induced fever of approximately 4-h duration. This effect was inhibited by pretreatment with IND and anti-IL-1beta Ab, but was not inhibited by pretreatment with LNA, GEN or CHE into the ventricle. These findings demonstrate that in the central nervous system, LPS increases IL-1beta production after activation of tyrosine kinase and NO synthase, and IL-1beta promotes prostaglandin production resulting in increased rectal temperature. Activation of tyrosine kinase in the central nervous system is probably a trigger for the febrile response induced by LPS.