Bacterial lipopolysaccharide fever is initiated via Toll-like receptor 4 on hematopoietic cells.

Lipopolysaccharide (LPS), a well-known bacterial pyrogen, is recognized by several receptors, including the Toll-like receptor 4 (TLR4), on various cells. Which of these receptors and cells are linked to fever production is unknown. By constructing 4 mouse chimeras and studying their thermoregulatory responses, we found that all 3 phases of the typical LPS fever depend on TLR4 signaling. The first phase is triggered via the TLR4 on hematopoietic cells. The second and third phases involve TLR4 signaling in both hematopoietic and nonhematopoietic cells.

Expanding the febrigenic role of cyclooxygenase-2 to the previously overlooked responses.

Previous studies on the role of cyclooxygenase (COX)-1 and -2 in fever induced by intravenous LPS have failed to investigate the role of these isoenzymes in the earliest responses: monophasic fever (response to a low, near-threshold dose of LPS) and the first phase of polyphasic fever (response to higher doses). We studied these responses in 96 mice that were COX-1 or COX-2 deficient (-/-) or sufficient (+/+). Each mouse was implanted with a temperature telemetry probe into the peritoneal cavity and a jugular catheter. The study was conducted at a tightly controlled, neutral ambient temperature (31 degrees C). To avoid stress hyperthermia (which masks the onset of fever), all injections were performed through a catheter extension. The +/+ mice responded to intravenous saline with no change in deep body temperature. To a low dose of LPS (1 microg/kg iv), they responded with a monophasic fever. To a higher dose (56 microg/kg), they responded with a polyphasic fever. Neither monophasic fever nor the first phase of polyphasic fever was attenuated in the COX-1 -/- mice, but both responses were absent in the COX-2 -/- mice. The second and third phases of polyphasic fever were also missing in the COX-2 -/- mice. The present study identifies a new, critical role for COX-2 in the mediation of the earliest responses to intravenous LPS: monophasic fever and the first phase of polyphasic fever. It also suggests that no product of the COX-1 gene, including the splice variant COX-1b (COX-3), is essential for these responses.

Thermoregulatory responses to lipopolysaccharide in the mouse: dependence on the dose and ambient temperature.

Most published studies of thermoregulatory responses of mice to LPS involved a stressful injection of LPS, were run at a poorly controlled and often subneutral ambient temperature (T(a)), and paid little attention to the dependence of the response on the LPS dose. These pitfalls have been overcome in the present study. Male C57BL/6 mice implanted with jugular vein catheters were kept in an environmental chamber at a tightly controlled T(a). The relationship between the T(a)s used and the thermoneutral zone of the mice was verified by measuring tail skin temperature, either by infrared thermography or thermocouple thermometry. Escherichia coli LPS in a wide dose range (10(0)-10(4) microg/kg) was administered through an extension of the jugular catheter from outside the chamber. The responses observed were dose dependent. At a neutral T(a), low (just suprathreshold) doses of LPS (10(0)-10(1) microg/kg) caused a monophasic fever. To a slightly higher dose (10(1.5) microg/kg), the mice responded with a biphasic fever. To even higher doses (10(1.75)-10(4) microg/kg), they responded with a polyphasic fever, of which three distinct phases were identified. The dose dependence and dynamics of LPS fever in the mouse appeared to be remarkably similar to those seen in the rat. However, the thermoregulatory response of mice to LPS in a subthermoneutral environment is remarkably different from that of rats. Although very high doses of LPS (10(4) microg/kg) did cause a late (latency, approximately 3 h) hypothermic response in mice, the typical early (latency, 10-30 min) hypothermic response seen in rats did not occur. The present investigation identifies experimental conditions to study LPS-induced mono-, bi-, and polyphasic fevers and late hypothermia in mice and provides detailed characteristics of these responses.

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.

Lipopolysaccharide fever is initiated via a capsaicin-sensitive mechanism independent of the subtype-1 vanilloid receptor.

As pretreatment with intraperitoneal capsaicin (8-methyl-N-vanillyl-6-nonenamide, CAP), an agonist of the vanilloid receptor known as VR1 or transient receptor potential channel-vanilloid receptor subtype 1 (TRPV-1), has been shown to block the first phase of lipopolysaccharide (LPS) fever in rats, this phase is thought to depend on the TRPV-1-bearing sensory nerve fibers originating in the abdominal cavity. However, our recent studies suggest that CAP blocks the first phase via a non-neural mechanism. In the present work, we studied whether this mechanism involves the TRPV-1. Adult Long-Evans rats implanted with chronic jugular catheters were used. Pretreatment with CAP (5 mg kg(-1), i.p.) 10 days before administration of LPS (10 microg kg(-1), i.v.) resulted in the loss of the entire first phase and a part of the second phase of LPS fever. Pretreatment with the ultrapotent TRPV-1 agonist resiniferatoxin (RTX; 2, 20, or 200 microg kg(-1), i.p.) 10 days before administration of LPS had no effect on the first and second phases of LPS fever, but it exaggerated the third phase at the highest dose. The latter effect was presumably due to the known ability of high doses of TRPV-1 agonists to cause a loss of warm sensitivity, thus leading to uncontrolled, hyperpyretic responses. Pretreatment with the selective competitive TRPV-1 antagonist capsazepine (N-[2-(4-chlorophenyl)ethyl]-1,3,4,5-tetrahydro-7,8-dihydroxy-2H-2-benzazepine-2-carbothioamidem, CPZ; 40 mg kg(-1), i.p.) 90 min before administration of LPS (10 microg kg(-1), i.v.) or CAP (1 mg kg(-1), i.p.) did not affect LPS fever, but blocked the immediate hypothermic response to acute administration of CAP. It is concluded that LPS fever is initiated via a non-neural mechanism, which is CAP-sensitive but RTX- and CPZ-insensitive. The action of CAP on this mechanism is likely TRPV-1-independent. It is speculated that this mechanism may be the production of prostaglandin E(2) by macrophages in LPS-processing organs.

A new function of the leptin receptor: mediation of the recovery from lipopolysaccharide-induced hypothermia.

Obese (f/f) Koletsky rats lack the leptin receptor (LR), whereas their lean (F/?) counterparts bear a fully functional LR. By using f/f and F/? rats, we studied whether the LR is involved in lipopolysaccharide (LPS)-induced fever and hypothermia. The body temperature responses to LPS (10 or 100 microg/kg iv) were measured in Koletsky rats exposed to a thermoneutral (28 degrees C) or cool (22 degrees C) environment. Rats of both genotypes responded to LPS with fever at 28 degrees C and with dose-dependent hypothermia at 22 degrees C. The fever responses of the f/f and F/? rats were identical. The hypothermic response of the f/f rats was markedly prolonged compared with that of the F/? rats. The prolonged hypothermic response to LPS in the f/f rats was accompanied by enhanced NF-kappaB signaling in the hypothalamus and an exaggerated rise in the plasma concentration of tumor necrosis factor (TNF)-alpha. The f/f rats did not respond to LPS with an increase in the plasma concentration of corticosterone or adrenocorticotropic hormone, whereas their F/? counterparts did. The hypothermic response to TNF-alpha (80 microg/kg iv) was markedly prolonged in the f/f rats. These data show that the LR is essential for the recovery from LPS hypothermia. LR-dependent mechanisms of the recovery from LPS hypothermia include activation of the anti-inflammatory hypothalamo-pituitary-adrenal axis, inhibition of both the production and hypothermic action of TNF-alpha, and suppression of inflammatory (via NF-kappaB) signaling in the hypothalamus.

Febrigenic signaling to the brain does not involve nitric oxide.

1. The involvement of peripheral nitric oxide (NO) in febrigenic signaling to the brain has been proposed because peripherally administered NO synthase (NOS) inhibitors attenuate lipopolysaccharide (LPS)-induced fever in rodents. However, how the unstable molecule of NO can reach the brain to trigger fever is unclear. It is also unclear whether NOS inhibitors attenuate fever by blocking febrigenic signaling or, alternatively, by suppressing thermogenesis in brown fat. 2. Male Wistar rats were chronically implanted with jugular catheters; their colonic and tail skin temperatures (T(c) and T(sk)) were monitored. 3. Study 1 was designed to determine whether the relatively stable, physiologically relevant forms of NO, that is, S-nitrosoalbumin (SNA) and S-nitrosoglutathione (SNG), are pyrogenic and whether they enhance LPS fever. At a neutral ambient temperature (T(a)) of 31 degrees C, afebrile or LPS (1 microg kg(-1), i.v.)-treated rats were infused i.v. with SNA (0.34 or 4.1 micromol kg(-1); the controls received NaNO(2) and albumin) or SNG (10 or 60 micromol kg(-1); the controls received glutathione). T(c) of SNA- or SNG-treated rats never exceeded that of the controls. 4. In Study 2, we tested whether the known fever-attenuating effect of the NOS inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME) at a subneutral T(a) (when fever is brought about by thermogenesis) also occurs at a neutral T(a) (when fever is brought about by skin vasoconstriction). At a subneutral T(a) of 24 degrees C, L-NAME (2.5 mg kg(-1), i.v.) attenuated LPS (10 microg kg(-1), i.v.) fever, presumably by inhibiting thermogenesis. At 31 degrees C, L-NAME enhanced LPS fever by augmenting skin vasoconstriction (T(sk) fall). 5. In summary, both SNA and SNG had no pyrogenic effect of their own and failed to enhance LPS fever; peripheral L-NAME attenuated only fever brought about by increased thermogenesis. It is concluded that NO is uninvolved in febrigenic signaling to the brain.