Kupffer cell-generated PGE2 triggers the febrile response of guinea pigs to intravenously injected LPS

Because the onset of fever induced by intravenously (i.v.) 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, PGE2 could 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 microg/kg body wt i.v.) into conscious guinea pigs and measured their plasma levels of LPS, PGE2, TNF-alpha, IL-1beta, and IL-6 before and 15, 30, 60, 90, and 120 min after LPS administration; Tc was 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 Tc that was temporally correlated with the elevation of plasma PGE2. KC depletion prevented the Tc and plasma PGE2 rises and slowed the clearance of LPS from the blood. TNF-alpha was not detectable in plasma until 30 min and in IL-1beta 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-alpha, the maximal level of which was increased approximately twofold in the KC-depleted animals. In a follow-up experiment, PGE2 antiserum administered i.v. 10 min before LPS significantly attenuated the febrile response to LPS. Together, these results support the view that, in guinea pigs, PGE2 rather than pyrogenic cytokines is generated by KCs in immediate response to i.v. LPS and triggers the febrile response.     

The cytokine-prostaglandin cascade in fever production: fact or fancy?

(1) The classical view of the genesis of infectious fevers is that they develop in sequential steps, starting with the production by peripheral mononuclear phagocytes activated by the infectious noxa (i.e., the invading pathogens and/or their products, e.g., bacterial endotoxic lipopolysaccharides (LPS)) of pyrogenic cytokines, principally tumor necrosis factor-, interleukin(IL)-1β and IL-6, the release of these cytokines into the bloodstream, their transport to targets accessible from blood (e.g., cerebral microvessels, the organum vasculosum laminae terminalis) in close proximity to the ventromedial preoptic area (VMPO, the presumptive brain site of the febrigenic controller), and the consequent generation of stimulatory signals directed to the VMPO. An alternative view is that the message of the pyrogenic cytokines elaborated in the periphery is conveyed to the VMPO via a neural rather than a humoral pathway. In both views, cyclooxygenase (COX)-2-dependent prostaglandin (PG) E₂ is considered to be the proximal fever mediator induced in the VMPO by these cytokines, modulating the activity of thermosensitive neurons contained in this region and effecting the development of fever. (2) However, peripheral cytokines are not consistently detectable in febrile illnesses, and it was recently reported that neither circulating LPS nor cytokine levels are increased at the onset of robust fevers induced in rats by subcutaneous injections of replicating Escherichia coli. (3) And it was reported long ago that the intracerebroventricular (icv) injection of PGE₂ did not evoke fever in newborn lambs and goats although these animals responded normally to the intravenous injection of LPS, and that the icv-injection of synthetic PGE₂ antagonists that prevented the febrile response of rabbits to icv-injected PGE₂ did not inhibit that to a simultaneously injected endogenous pyrogen. (4) Other, more recent data indicate that the pyrogenic chemokine macrophage inflammatory protein-1β produces fever independently of PGE₂. (5) We found recently that the febrile response of adult guinea pigs to intravenously injected LPS is initiated significantly before the appearance of cytokines in the blood and, moreover, is evoked and sustainable in the absence of preoptic PGE₂. (6) What is amiss? Are the contradictory data fallacious or should the conventional wisdom be revisited? This issue is considered in this article and an explanation suggested for these disparate findings.     

Preoptic nitric oxide attenuates endotoxic fever in guinea pigs by inhibiting the POA release of norepinephrine

Lipopolysaccharide (LPS) administration induces hypothalamic nitric oxide (NO); NO is antipyretic in the preoptic area (POA), but its mechanism of action is uncertain. LPS also stimulates the release of preoptic norepinephrine (NE), which mediates fever onset. Because NE upregulates NO synthases and NO induces cyclooxygenase (COX)-2-dependent PGE(2), we investigated whether NO mediates the production of this central fever mediator. Conscious guinea pigs with intra-POA microdialysis probes received LPS intravenously (2 mug/kg) and, thereafter, an NO donor (SIN-1) or scavenger (carboxy-PTIO) intra-POA (20 mug/mul each, 2 mul/min, 6 h). Core temperature (T(c)) was monitored constantly; dialysate NE and PGE(2) were analyzed in 30-min collections. To verify the reported involvement of alpha(2)-adrenoceptors (AR) in PGE(2) production, clonidine (alpha(2)-AR agonist, 2 mug/mul) was microdialyzed with and without SIN-1 or carboxy-PTIO. To assess the possible involvement of oxidative NE and/or NO products in the demonstrated initially COX-2-independent POA PGE(2) increase, (+)-catechin (an antioxidant, 3 mug/mul) was microdialyzed, and POA PGE(2), and T(c) were determined. SIN-1 and carboxy-PTIO reduced and enhanced, respectively, the rises in NE, PGE(2), and T(c) produced by intravenous LPS. Similarly, they prevented and increased, respectively, the delayed elevations of PGE(2) and T(c) induced by intra-POA clonidine. (+)-Catechin prevented the LPS-induced elevation of PGE(2), but not of T(c). We conclude that the antipyretic activity of NO derives from its inhibitory modulation of the LPS-induced release of POA NE. These data also implicate free radicals in POA PGE(2) production and raise questions about its role as a central LPS fever mediator.     

Preoptic norepinephrine mediates the febrile response of guinea pigs to lipopolysaccharide

Norepinephrine (NE) microdialyzed in the preoptic area (POA) raises core temperature (T(c)) via 1) alpha(1)-adrenoceptors (AR), quickly and independently of POA PGE(2), and 2) alpha(2)-AR, after a delay and PGE(2) dependently. Since systemic lipopolysaccharide (LPS) activates the central noradrenergic system, we investigated whether preoptic NE mediates LPS fever. We injected LPS (2 microg/kg iv) in guinea pigs prepared with intra-POA microdialysis probes and determined POA cerebrospinal (CSF) NE levels. We similarly microdialyzed prazosin (alpha(1) blocker, 1 microg/microl), yohimbine (alpha(2) blocker, 1 microg/microl), SC-560 [cyclooxygenase (COX)-1 blocker, 5 microg/microl], acetaminophen (presumptive COX-1v blocker, 5 microg/microl), or MK-0663 (COX-2 blocker, 0.5 microg/microl) in other animals before intravenous LPS and measured CSF PGE(2). All of the agents were perfused at 2 microg/min for 6 h. T(c) was monitored constantly. POA NE peaked within 30 min after LPS and then returned to baseline over the next 90 min. T(c) increased within 12 min to a first peak at approximately 60 min and to a second at approximately 150 min and then declined over the following 2.5 h. POA PGE(2) followed a concurrent course. Prazosin pretreatment eliminated the first T(c) rise but not the second; PGE(2) rose normally. Yohimbine pretreatment did not affect the first T(c) rise, which continued unchanged for 6 h; the second rise, however, was absent, and PGE(2) levels did not increase. SC-560 and acetaminophen did not alter the LPS-induced PGE(2) and T(c) rises; MK-0663 prevented both the late PGE(2) and T(c) rises. These results confirm that POA NE is pivotal in the development of LPS fever.     

Site-specific modulation of LPS-induced fever and interleukin-1 beta expression in rats by interleukin-10

Bacterial lipopolysaccharide (LPS) induces fever that is mediated by pyrogenic cytokines such as interleukin (IL)-1 beta. We hypothesized that the anti-inflammatory cytokine IL-10 modulates the febrile response to LPS by suppressing the production of pyrogenic cytokines. In rats, intravenous but not intracerebroventricular infusion of IL-10 was found to attenuate fever induced by peripheral administration of LPS (10 microg/kg iv). IL-10 also suppressed LPS-induced IL-1 beta production in peripheral tissues and in the brain stem. In contrast, central administration of IL-10 attenuated the febrile response to central LPS (60 ng/rat icv) and decreased IL-1 beta production in the hypothalamus and brain stem but not in peripheral tissues and plasma. Furthermore, intravenous LPS upregulated expression of IL-10 receptor (IL-10R1) mRNA in the liver, whereas intracerebroventricular LPS enhanced IL-10R1 mRNA in the hypothalamus. We conclude that IL-10 modulates the febrile response by acting in the periphery or in the brain dependent on the primary site of inflammation and that its mechanism of action most likely involves inhibition of local IL-1 beta production.     

Fever-inducing sympathetic neural pathways

1. Fever is the widely known hallmark of disease and induced by the action of the nervous system. 2. It is generally accepted that the action of prostaglandin E₂ in the preoptic area (POA) triggers the stimulation of the sympathetic nervous system, and this results in the production of fever. 3. Recent findings have demonstrated that the rostral medullary raphe regions are essential for the pyrogenic signal transmission from the POA to sympathetic output neurons. 4. Here, I review the functional role of these raphe regions in febrile responses and also discuss the pyrogenic transmission from the POA to the raphe regions.     

Blunted febrile response to intravenous endotoxin in starved rats

The effects of fasting on the febrile responses to intravenous injection of bacterial lipopolysaccharide (LPS; endotoxin) of Escherichia coli were investigated in rats. Ad libitum-fed rats (C) produced a biphasic fever with an increase in the temperature difference between brown adipose tissue and colon and shivering activity (SA). Measurement by a direct calorimeter showed no particular changes in heat loss. Rats starved for 4 days (F4) responded to intravenous LPS with a monophasic fever accompanied by an increase in SA only. However the maximal rise in colonic temperature (Tco) did not differ from C rats. Subsequent 2-day fasting reduced SA and the maximal fever height. Endogenous pyrogen (EP) injected intravenously produced a prompt rise in Tco followed by prolonged hyperthermia in C rats. In the F4 rats, there was no such sustained rise in Tco as a result of intravenous EP. The response in Tco to intravenous prostaglandin E2 (PGE2) was the same in fed and starved rats. The administration of LPS, EP, and PGE2 into the lateral ventricle evoked a similar extent of hyperthermia in C and F4 rats. Because the second phase of fever has been shown to occur after pyrogens are translated into a febrile stimulus within the blood-brain barrier, it is assumed that the functional changes of the blood-brain barrier such as in the permeability of pyrogens or in the sensitivity of pyrogen receptors resulted in the absence of the second phase of fever in starved rats.     

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.     

Cells That Trigger Fever

Bacterial lipopolysaccharide (LPS) is recognized by several receptors, including the toll-like receptor (TLR) 4, on various cells. Among many biological responses to LPS is fever, an often polyphasic rise in body temperature that is thought to be mediated by prostaglandin (PG) E2. Which receptors on which cells are linked to fever production is unknown. It is also unknown which cells produce PGE2 that triggers the earliest (first) phase of fever. Two recent studies from our group answer these questions. In the first one, we studied LPS-induced fever in mouse chimeras selectively lacking the TLR4 in hematopoietic or nonhematopoietic cells. We found that the first phase of fever is triggered via the TLR4 on hematopoietic cells. In the second study, we investigated LPS fever in rats. We found that the number of cells expressing cyclooxygenase (COX)-2, a PGE2-synthesizing enzyme, surged at the onset of fever in the lung and liver (but not in the brain), and that most of these cells were macrophages. Because LPS-induced PGE2 production in macrophages is TLR4-dependent, it is tempting to speculate that the TLR4-bearing, bone marrow-derived cells implicated in fever pathogenesis by the first study are the same as the COX-2-positive macrophages identified in the second study. Hence, pulmonary and hepatic macrophages that recognize LPS via the TLR4 and rapidly produce PGE2 are likely triggers of the fever response.     

Brain IL-6- and PG-dependent actions of IL-1β and lipopolysaccharide in avian fever

There is no persuasive evidence of a correlation between proinflammatory cytokines and avian fever. In this study, for the first time, we use avian cytokines to investigate a role for proinflammatory cytokines in the central component of avian fever. IL-1β and IL-6 injected intracerebroventricularly into Pekin ducks (n = 8) initiated robust fevers of equal magnitude and duration, although there was a significant difference in the latency to a febrile response. In addition, the IL-1β-induced fever could be abolished with an intracerebroventricular injection of antibodies to avian IL-6 or an oral administration of a PG synthesis inhibitor. Our findings indicate the following sequence of events within the central component of the avian febrile mechanism: IL-1β gives rise to bioactive IL-6, which stimulates an accelerated synthesis of PGs, and these PGs then adjust the sensitivity of warm-sensitive neurons in the avian brain stem to mediate fever. Yet PGE? was not upregulated in the cerebrospinal fluid of ducks made febrile with LPS. We conclude that IL-1β and IL-6 may well mediate fever by instigating an accelerated synthesis of brain-derived PG, of a class other than PGE?, or that IL-6 serves as one of the terminal mediators of the avian febrile response.     

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

This study evaluated the participation of mu-opioid-receptor activation in body temperature (T(b)) 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 mu-receptor-antagonist cyclic d-Phe-Cys-Try-d-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP; 0.1-1.0 microg) reduced fever induced by LPS (5.0 microg/kg) but did not change T(b) at ambient temperatures of either 20 degrees C or 28 degrees C. The subcutaneous, intracerebroventricular, and intrahypothalamic injection of morphine (1.0-10.0 mg/kg, 3.0-30.0 microg, and 1-100 ng, respectively) produced a dose-dependent increase in T(b). Intracerebroventricular morphine also produced a peripheral vasoconstriction. Both effects were abolished by CTAP. CTAP (1.0 microg icv) reduced the fever induced by intracerebroventricular administration of TNF-alpha (250 ng), IL-6 (300 ng), CRF (2.5 microg), endothelin-1 (1.0 pmol), and macrophage inflammatory protein (500 pg) and the first phase of the fever induced by PGF(2alpha) (500.0 ng) but not the fever induced by IL-1beta (3.12 ng) or PGE(2) (125.0 ng) or the second phase of the fever induced by PGF(2alpha). 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 PGE(2) 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-1beta and prostaglandins) recruit the opioid system to cause a mu-receptor-mediated fever.     

Possible sequence of pyrogenic afferent processing in the POA

(1) It is generally considered that fever is modulated in the preoptic-anterior hypothalamic area (POA) in response to signaling by pyrogenic cytokines elaborated in the periphery by mononuclear phagocytes and the consequent induction of prostaglandin (PG)E₂ in the POA. The mechanism of the centripetal transmission of this pyrogenic signal, however, is controversial. One hypothesis suggests that it is conveyed via the vagus to the nucleus tractus solitarius and from there to the POA via the ventral noradrenergic bundle, causing the intraPOA release of norepinephrine (NE) which then stimulates the production of PGE₂. (2) In this article, we review recent data from our laboratory showing that NE microdialyzed into the POA of conscious guinea pigs or injected intracerebroventricularly into conscious mice indeed evokes two distinct core temperature (T_c) rises, viz., one ₁-adrenoceptor (AR)-mediated, rapid in onset and PGE₂-independent, and the other ₂-AR-mediated, delayed and COX-2/PGE₂-dependent. (3) We further present new data suggesting that the febrile response of conscious guinea pigs to intraperitoneally injected lipopolysaccharide (LPS) is mediated by intraPOA NE in accord with the above sequence, i.e., via ₁-AR to initiate the first, PGE₂-independent elevation of T_c, and via ₂-AR to induce the delayed production of COX-2-dependent PGE₂ and the continued rise of T_c. (4) These results thus validate the presumptive involvement of NE in LPS fever induction in guinea pigs.     

Preoptic alpha 1- and alpha 2-noradrenergic agonists induce, respectively, PGE2-independent and PGE2-dependent hyperthermic responses in guinea pigs

We have shown previously that norepinephrine (NE) microdialyzed into the preoptic area (POA) of conscious guinea pigs stimulates local PGE(2) release. To identify the cyclooxygenase (COX) isozyme that catalyzes the production of this PGE(2) and the adrenoceptor (AR) subtype that mediates this effect, we microdialyzed for 6 h NE, cirazoline (alpha(1)-AR agonist), and clonidine (alpha(2)-AR agonist) into the POA of conscious guinea pigs pretreated intrapreoptically (intra-POA) with SC-560 (COX-1 inhibitor) or nimesulide or MK-0663 (COX-2 inhibitors) and measured the animals' core temperature (T(c)) and intra-POA PGE(2) responses. Cirazoline induced T(c) rises promptly after the onset of its dialysis without altering PGE(2) levels. NE and clonidine caused early falls followed by late rises of T(c); intra-POA PGE(2) levels were closely correlated with this thermal course. COX-1 inhibition attenuated the clonidine-induced T(c) and PGE(2) falls but not the NE-elicited hyperthermia, but COX-2 inhibition suppressed both the clonidine- and NE-induced T(c) and PGE(2) rises. Coinfused cirazoline and clonidine reproduced the late T(c) rise of clonidine but not its early fall and also not the early rise produced by cirazoline; on the other hand, the PGE(2) responses were similar to those to NE. Prazosin (alpha(1)-AR antagonist) and yohimbine (alpha(2)-AR antagonist) blocked the effects of their respective agonists. These results indicate that alpha(1)- and alpha(2)-AR agonists microdialyzed into the POA of conscious guinea pigs evoke distinct T(c) responses: alpha(1)-AR activation produces quick, PGE(2)-independent T(c) rises, and alpha(2)-AR stimulation causes an early T(c) fall and a late, COX-2/PGE(2)-dependent T(c) rise.     

Threshold dissociation of thermoregulatory effector responses in febrile rabbits

When the core temperature stabilizes at a hyperthermic level after iv injection of lipopolysaccharide (LPS), the threshold core temperature for cutaneous vasoconstriction (Thcv) is significantly increased in hot and neutral environments, while the threshold core temperature for shivering (Thsh) is not significantly altered in hot or cold environments but is significantly reduced at thermoneutrality. This type of dissociated threshold alterations of thermoregulatory effector responses seems to be typical for the febrile response of rabbits to LPS. Because the same threshold dissociation can be demonstrated after icv injection of LPS, the systemic and the central effects of LPS in the generation of fever seem to be mediated by identical mechanisms. Prostaglandins of the E series (PGE), one of the mediators considered as important in fever generation, cause parallel increases in Thcv and Thsh when injected icv. This indicates that the mode of action of PGE on the central targets producing hyperthermia differs from that of the ensemble of mediators involved in the generation of LPS fever in rabbits. In rabbits pretreated with aspirin, the threshold dissociation after iv LPS injection still occurs. This indicates that factors other than PGE play an important role in the generation of the threshold dissociation of thermoregulatory effector responses, which is typical for LPS fever. These data indicate also that the states of activity of the thermoregulatory effectors involved in the febrile responses can be altered individually and that the activities of these effectors during LPS fever are quite different from their activities in the control state.     

Heme oxygenase-1 induction by cobalt protoporphyrin enhances fever and inhibits pyrogenic tolerance to lipopolysaccharide

Heme oxygenase-1 (HO-1) is an enzyme that catalyzes degradation of the heme and regulates its availability for newly synthetized hemeproteins such as cyclooxygenases, NO synthases and cytochrome P450. Moreover, HO-1 activity modulates synthesis of cytokines and prostaglandins. All of these factors are well-defined components of fever and pyrogenic tolerance mechanisms. We examine the effect of HO-1 induction and activation using cobalt protoporphyrin (CoPP) on changes in body temperature (T_b), plasma levels of interleukin-6 (IL-6), prostaglandin E₂ (PGE₂) and HO-1 protein in the course of these processes. Intraperitoneally (i.p.) pre-treatment of rats with CoPP (5 mg kg⁻¹) significantly accelerated and enhanced the early stage of lipopolysaccharide (LPS)-induced fever and shortened a post-fever recovery to normal temperature. Pre-treatment with CoPP significantly potentiated the increase in plasma IL-6, PGE₂ and HO-1 levels measured 4 h after the LPS administration. Furthermore, induction of HO-1 attenuated the development of pyrogenic tolerance to repeated injections of LPS. Based on these data we conclude that heme oxygenase-1 may act as a physiological regulator of the febrile response intensity to bacterial infections.     

Characterization of the febrile response induced by fibroblast-stimulating lipopeptide-1 in guinea pigs

Recently, it has been shown that the Toll-like receptors-2 and -6 agonist fibroblast-stimulating lipopeptide-1 (FSL-1) have the capacity to induce fever and sickness behavior in rats. Since the mechanisms of the fever-inducing effects of FSL-1 are still unknown, we tested the pyrogenic properties of FSL-1 in guinea pigs and assessed a role for TNF-alpha and prostaglandins in the manifestation of the febrile response to this substance. Intra-arterial and intraperitoneal injections of FSL-1 caused dose-dependent fevers that coincided with elevated plasma levels of TNF and IL-6, the intraperitoneal route of administration being more effective than the intra-arterial route. Intra-arterial or intraperitoneal injection of a soluble form of the TNF type 1 receptor, referred to as TNF binding protein (TNFbp), together with FSL-1, completely neutralized FSL-1-induced circulating TNF and reduced fever and circulating IL-6. Intra-arterial or intraperitoneal injection of the nonselective cyclooxygenase (COX)-inhibitor diclofenac depressed fever and FSL-1-induced elevations of circulating PGE2. Circulating TNF and IL-6, however, remained unimpaired by treatment with diclofenac. In conclusion, FSL-1-induced fever in guinea pigs depends, in shape and duration, on the route of administration and is, to a high degree, mediated by pyrogenic cytokines and COX products.     

Vagal CCK and 5-HT(3) receptors are unlikely to mediate LPS or IL-1beta-induced fever

Previous studies suggested that peripheral immune mediators may involve intermediates acting on the vagus nerve, such as CCK or serotonin (5-HT). We have therefore investigated a possible role for vagal CCK-A and 5-HT(3) receptors in the febrile response after intraperitoneal human recombinant interleukin-1beta (IL-1beta) or lipopolysaccharide (LPS). Unanesthetized, adult male rats instrumented with abdominal thermistors were given intraperitoneal CCK-8 sulfate (100 or 150 microgram/kg) or 2-methyl-5-hydroxytryptamine maleate (4 mg/kg). In other experiments, rats were treated with either antagonists to the 5-HT(3) receptor (ondansetron HCl; 100 microgram/kg) or the CCK-A receptor (L-364,718, 100 or 200 microgram/kg) in combination with LPS or IL-1beta. CCK administration caused a short-lived hypothermia, but interference with the action of endogenous CCK at CCK-A receptors was without effect on IL-1beta- or LPS-induced fever. Neither activation of 5-HT(3) receptors nor blockade of 5-HT(3) receptors affected body temperature or LPS fever. Taken together, our data support the idea that vagal afferents responsive to pyrogenic cytokines may be different from those responsive to CCK or 5-HT.     

In vivo evidence that the rise in plasma IL 6 following injection of a fever-inducing dose of LPS is mediated by IL 1 beta

Although it has often been speculated that Interleukin (IL) 1 alpha and IL 1 beta are circulating endogenous pyrogens (EP), there are few data demonstrating an elevation of these cytokines in the plasma of febrile animals. We hypothesized that IL 1 is released locally and may act to stimulate the release of another pyrogen, IL 6, which circulates to the brain to cause fever. The major purpose of the present study was to determine whether pretreatment of rats with antiserum to IL 1 beta, which attenuates lipopolysaccharide (LPS) induced fever, also results in an attenuation of the rise in plasma and cerebrospinal fluid (CSF) concentrations of IL 6. Our results show that injection of IL 1 beta produced dose-dependent rises in temperature and increases in plasma and CSF IL 6 activity, and that pretreatment of rats i.v. with antiserum to IL 1 beta produced a 55% decrease in the fever caused by LPS injection, a 68% decrease in plasma IL 6, and a 67% decrease in CSF IL 6. These data confirm the findings of previous studies that IL 1 beta is required for a portion of LPS-induced fever and also provide the first in vivo demonstration that the rise of IL 6 in rats injected with a fever-inducing dose of LPS can be significantly blocked by antiserum to IL 1 beta. Overall, the data in our study can be interpreted as being consistent with the hypothesis that the pyrogenic effect of IL 1 beta is mediated mainly through the release of IL 6, but conclusive confirmation of this hypothesis must await studies with antibodies to IL 6.