Fever in rats during normal and dehydrated conditions

The effect of endogenous pyrogen (EP, from rabbit) and endotoxin (Salmonella typhosa) on rectal temperature (Tre) was investigated in normal and dehydrated rats of both sexes. Intraperitoneal injection of either EP or endotoxin did not affect body temperature. In addition, no changes in Tre were observed when endotoxin was injected intravenously in normally hydrated male rats, but significant falls in Tre occurred in normal female rats. However, intravenous injection of EP produced fever in both sexes, but females generally showed smaller responses. A second intravenous injection of endotoxin, given 3 days after the first injection, always produced fever in normally hydrated rats. The pattern of this febrile response was monophasic. In contrast to the response in normal rats, intravenous endotoxin produced significant fevers with a biphasic pattern in dehydrated rats of either sex, but the febrile responses of male rats were greater than those of female rats. On the other hand, there were no significant differences between febrile responses to intravenous EP exhibited by normal and dehydrated animals. These results show that rats of both sexes possess physiological mechanisms capable of producing a fever following intravenous injections of EP.     

Effect ofE. coli endotoxin on temperature,oxygen consumption and brown adipose tissue thermogenesis in rats and mice

The effects ofE. coli endotoxin 0127 B8 on oxygen consumption, temperature, and on the activity of the proton conductance pathway in brown adipose tissue (BAT) were investigated in rats and mice. In rats an increase was observed in rectal and skin temperature, whole body oxygen consumption and GDP binding in BAT. In mice only the rise in rectal and skin temperature were significantly changed by endotoxin administration.These findings suggest that in some species BAT is involved in the production of endotoxin induced fever and increased energy expenditure.     

Endotoxin-challenges precocial neonates and iron changes

In adult mammals fever is associated with the reduction of blood plasma iron level. Immature mammals, however, show either a decrease (precocial animals such as guinea pig neonates) or a lack of reduction (altricial animals such as human neonates) of plasma iron in response to endotoxin. In order to determine whether this difference is connected with maturity just after delivery, plasma iron concentration, hematocrit, body temperature and body mass were measured in rat pups injected with E. coli endotoxin in doses of 50 or 200 μg kg⁻¹. Rat pups, like human neonates, are altricial animals. In 7-day-old rats injection of LPS led to a dose-dependent decrease in plasma iron level. The fall in plasma iron was accompanied by changes in body temperature and body mass. The results showed that plasma iron response to endotoxin in altricial rat neonates is similar to that observed in precocial guinea pig pups. Accepted: 4 October 1996     

Neutralization of macrophage inflammatory protein-2 blocks the febrile response induced by lipopolysaccharide in rats

1. This study was aimed to test the hypothesis that macrophage inflammatory protein-2 (MIP-2), a powerful chemotactic cytokine for neutrophils, plays a role in bacterial endotoxin fever.

2. The effect of specific anti-rat MIP-2 antibodies on lipopolysaccharide (LPS)-induced fever was tested. Intraperitoneal injection of LPS resulted in a biphasic fever and a significant increase in serum MIP-2 and prostaglandin (PG) E₂ levels which correlated with the start of fever. Intraperitoneal anti-MIP-2 (500 μg/kg) did not affect the body core temperature of unrestrained rats, but markedly attenuated LPS-induced fever.

3. Treatment with the cyclooxygenase inhibitor ibuprofen (10 mg/kg) resulted in a significant attenuation of LPS-induced fever and a significant decrease of MIP-2 and PGE₂ production.

4. These results indicate that LPS fever in rats is, at least, in part dependent on mechanisms involving neutrophils chemotaxis, and that MIP-2 may be an important mediator in the genesis of fever via prostaglandin-dependent pathways.     

Angiotensin-converting enzyme inhibitor inhibits dehydration-enhanced fever induced by endotoxin in rats

It has been reported that a host develops a marked fever under dehydrated conditions compared with normally hydrated conditions (11). The present study was carried out to investigate whether ANG II is involved in the enhancement seen in dehydrated rats of the fever induced by bacterial endotoxin. The results showed that intravenous injection of bacterial endotoxin produced a fever in dehydrated rats (rats deprived of water for 24 h) that was significantly greater than that seen in normally hydrated rats. In contrast, dehydration had no effect on the fever induced by intravenous interleukin-1beta (IL-1beta). Under dehydrated conditions, the enhanced endotoxin-induced fever was significantly inhibited by the angiotensin-converting enzyme inhibitor lisinopril, but the IL-1beta fever was not. These results suggest that the dehydration-induced enhancement of endotoxin fever is due, at least in part, to the action of ANG II, which elicits an increased production of pyrogenic cytokines such as IL-1.     

Endotoxin fever in the newborn kitten. The role of prostaglandins and monoamines.

In 5--10 day-old kittens at thermoneutral environmental temperature cerebroventricular injections of 10 microgram serotonin or noradrenaline caused hyperthermia and hypothermia, respectively. Central injections of 20 and 200 ng prostaglandin E1 induced hyperthermia. Monophasic fever followed the cerebroventricular injections of 0.2 or 0.002 microgram E. coli endotoxin, both in thermoneutral and moderately cool environments. In kittens pretreated with para-chlorophenylalanine (PCPA) the endotoxin induced rise in body temperature was attenuated within 60 to 90 min after the endotoxin. Indomethacin pretreatment prevented the first part of the febrile response and only a slight temperature rise occurred after a long latency. Central injections of phentolamine did not modify the fever response, while centrally applied propranolol modified the fever course so that it resembled that seen in PCPA treated kittens. The central mediation of endotoxin fever in the kitten is complex, despite that the pattern of the temperature change is simple (monophasic). Arachidonic acid metabolites and serotonin of the central nervous system may be involved in the reaction, while the activation of central noradrenergic mechanisms does not seem to be indispensable for the response. The changes in mediators are similar to those in newborn guinea pigs, although the fever course is different in the two species.     

Dynamics of endotoxin fever in the rabbit

To analyze the dynamic properties of body temperature and effector mechanisms during endotoxin fever, both experimental and mathematical procedures were applied. Experiments were carried out on rabbits in a climatic chamber at various ambient temperatures. Salmonella typhosa endotoxin (0.1 microgram/kg) was injected into an ear vein. A biphasic core temperature increase evoked by different effector mechanisms depending on ambient temperature was observed. A mathematical model based on experimental results with nonfebrile rabbits predicts the effector behavior at all ambient temperatures. From a comparison of experimental results with the model prediction, it is concluded that the increase of core temperature during fever is essentially caused by a dynamic shift of the controller characteristics. The effect of the pyrogen may be simulated by a resultant fever-controlling signal that is biphasic but increases more steeply than does core temperature. The analysis suggests that the three possible fever-driving effectors, metabolism, ear blood flow, and respiratory evaporative heat loss, should be controlled by the same resultant signal, although the time courses of the effectors and of core temperature vary distinctly at different air temperatures. The model uses an additive controller structure.     

常用大鼠发热模型研究

目的探讨干酵母、2,4-二硝基酚、脂多糖（LPS）、细菌内毒素引起SD大鼠实验性发热的过程和特点,比较不同浓度外致热原对大鼠发热过程的影响。方法建立大鼠干酵母（2 g/kg、1 g/kg）、2,4-二硝基酚（30mg/kg、15 mg/kg）、LPS（100μg/kg、20μg/kg）、细菌内毒素（120 EU/kg、60 EU/kg）发热模型,记录不同时间点大鼠升温值,绘制各模型平均升温曲线,比较不同大鼠发热模型的发热特点。结果皮下注射干酵母混悬液,注射后2～3 h开始升温,6～7 h达峰值,升温持续20 h;皮下注射2,4-二硝基酚溶液,注射后20 min开始升温,1～1.5 h达峰值,升温持续3～5 h;腹腔注射LPS、细菌内毒素,注射后30 min开始升温,此后升温曲线表现为双相热或三相热,升温持续5～8 h。结论不同外致热原所致SD大鼠发热的过程和特点不同;外致热原浓度不同,所致大鼠发热过程和特点不同。解热试验中,应根据受试药物本身特点选用合适的大鼠发热模型。     

Endotoxin fever in para-chlorophenylalanine (PCPA) treated newborn guinea pigs and kittens.

Guinea pigs aged 0–3 days responded to 0.2 μg icv. E. coli endotoxin with phasic changes in body temperature; two rises separated by a marked fall. This fall was diminished in PCPA treated animals. In 5–10 day-old kittens the otherwise monophasic endotoxin fever was modified by PCPA treatment; the rise was attenuated in the period corresponding to the transient temperature fall of guinea pigs.     

Nutritional state and endotoxin fever of newborn rabbits.

At thermoneutral environments 6-10 day-old well-fed rabbits responded to 20 microgram/kg I.V. E. coli endotoxin with biphasic fever: temperature peaks at 60 and 120-150, and a transient fall between 60 and 90 min after endotoxin injection. In rabbits starved for 24 hours, and in runt rabbits body temperature did not rise, but a decline started 60 min after endotoxin administration, corresponding to the transient fall observed in well-fed animals and continuing until about the 100-120th min; thereafter body temperature tended to stabilize at the low level.     

Energy balance and brown adipose tissue thermogenesis during chronic endotoxemia in rats

The effects of continuously administered endotoxin on 7-day energy balance were investigated in male rats. Three groups of rats were implanted with osmotic pumps; two groups received saline-filled pumps, whereas the third received endotoxin. One of the saline groups was pair fed to match the food intake of the endotoxemic rats. After 7 days, body energy and protein and fat contents of rats were determined together with the energy content of food and feces. Endotoxin infusion not only induced fever, but it also suppressed appetite and significantly decreased body weight gain. Metabolizable energy intake was reduced by approximately 20% in infected rats. Although protein and fat gains were lowest in the endotoxin group, there appeared to be a selective loss of protein when considered as percent of body weight. Percent body fat was unaltered between the groups. Energy expenditure considered in absolute (kJ) or body weight-independent (kJ/kg0.67) terms yielded similar patterns of results; expenditure (kJ) was 10 and 20% (P less than 0.05, P less than 0.01) lower in the endotoxemic and pair-fed rats, respectively, compared with controls. Hence, compared with pair-fed rats, endotoxin-infused animals had a 10% rise in their expenditure. Brown adipose tissue thermogenesis was assessed by mitochondrial binding of guanosine 5'-diphosphate, and results showed that binding was greatest in endotoxemic rats and lowest in the pair-fed animals. The present results suggest that in this endotoxemic model appetite suppression exacerbates changes in energy balance. However, the reduction in body weight gain is also dependent on a decrease in metabolic efficiency and an increase in total energy expenditure.(ABSTRACT TRUNCATED AT 250 WORDS)     

Immunoadjuvants enhance the febrile responses of rats to endogenous pyrogen

The febrile responses of male Sprague-Dawley rats to a semipurified endogenous pyrogen produced from human monocytes were characterized by establishing fever dose-response curves. The animals were then injected intravenously with a number of substances that possessed the common properties of stimulating the phagocytic activity of the cells of the reticuloendothelial system and of acting as immunoadjuvants. The substances used were zymosan, lipopolysaccharide endotoxin, and muramyl dipeptide. Three days after any of these immunoadjuvants were injected, the fever sensitivity of the rats was remeasured. In each case, the slope of the fever dose-response curve tripled, and in some instances the response threshold for fever response was reduced by factors of three to eight. Furthermore, the maximum increase in body temperature produced by the endogenous pyrogen was more than doubled after immunoadjuvant treatment. By contrast latex beads, which are also phagocytized by the cells of the reticuloendothelial system but do not subsequently increase their phagocytic index nor do they enhance immune responses, had no effect on the fever sensitivity of rats in response to endogenous pyrogen. In the light of these findings, it is suggested that the febrile responses of rats to endogenous pyrogen are mediated in some manner by cells that possess some of the properties of reticuloendothelial cells. The location of these putative cells must be close to the circulation, because the immunoadjuvants used in this study were, for the most part, large molecular weight molecules that could not cross the blood-brain barrier easily.     

Thermoregulatory responses of rats to conventional preparations of lipopolysaccharide are caused by lipopolysaccharide per se-- not by lipoprotein contaminants

LPS preparations cause a variety of body temperature (T(b)) responses: monophasic fever, different phases of polyphasic fever, and hypothermia. Conventional (c) LPS preparations contain highly active lipoprotein contaminants (endotoxin proteins). Whereas LPS signals predominantly via the Toll-like receptor (TLR) 4, endotoxin proteins signal via TLR2. Several TLR2-dependent responses of immunocytes to cLPS in vitro are triggered by endotoxin proteins and not by LPS itself. We tested whether any T(b) response to cLPS from Escherichia coli 055:B5 is triggered by non-TLR4-signaling contaminants. A decontaminated (d) LPS preparation (free of endotoxin proteins) was produced by subjecting cLPS to phenol-water reextraction. The presence of non-TLR4-signaling contaminants in cLPS (and their absence in dLPS) was confirmed by showing that cLPS (but not dLPS) induced IL-1beta expression in the spleen and increased serum levels of TNF-alpha and IL-1beta of C3H/HeJ mice; these mice bear a nonfunctional TLR4. Yet, both cLPS and dLPS caused cytokine responses in C3H/HeOuJ mice; these mice bear a fully functional TLR4. We then studied the T(b) responses to cLPS and dLPS in Wistar rats preimplanted with jugular catheters. At a neutral ambient temperature (30 degrees C), a low (0.1 microg/kg iv) dose of cLPS caused a monophasic fever, whereas a moderate (10 microg/kg iv) dose produced a polyphasic fever. In the cold (20 degrees C), a high (500 microg/kg iv) dose of cLPS caused hypothermia. All T(b) responses to dLPS were identical to those of cLPS. We conclude that all known T(b) responses to LPS preparations are triggered by LPS per se and not by non-TLR4-signaling contaminants of such preparations.     

Responses of baboons to traditionally pyrogenic agents

It is not clear whether baboons develop fever in response to endotoxin or other pyrogens. We injected various pyrogens intravenously in 12 unrestrained baboons (Papio ursinus) and measured their body temperature using intra-abdominal radiotelemeters. Serum iron concentration was also measured. The baboons developed fever after injection of killed Staphylococcus aureus (5 X 10(7) organisms/kg). No significant fever was measured after injection of lipopolysaccharide (Salmonella typhosa) (0.1, 8, 40, and 100 micrograms/kg), bovine serum albumin (4 mg/kg), killed Salmonella minnesota (5 X 10(7) organisms/kg), and killed Salmonella typhi (5 X 10(7) organisms/kg). A significant decrease in serum iron concentration was found only after injection of S. aureus and lipopolysaccharide, 100 micrograms/kg. The phagocytic synthesis of interleukin-1 following pyrogen stimulation in baboons and some other primates appears to differ from that in man and in nonprimates.     

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.     

Endotoxin-induced ATP depletion in thyrotoxic rats

Effect of endotoxin from E. coli on the ATP content in heart muscle, the liver and the kidney of thyrotoxic rats was studied. When endotoxin (200-400 micrograms) was intravenously injected to rats taking drinking water containing 2-7.5 micrograms T3 per ml, body temperature rose and the heart rate increased. At the same time, a marked decrease in the ATP content in heart muscle and the kidney was observed together with an increase in Na+-K+-ATPase activity. Such changes were not observed or seen only to a small extent in euthyroid rats after endotoxin administration. Endotoxin-induced ATP depletion in T3-treated rats was prevented by administration of 5 mg hydrocortisone just prior to endotoxin injection. These findings indicate that endotoxin easily causes ATP depletion in some tissue or organs in thyrotoxicosis, even if the dose of endotoxin is not enough to produce such an effect in the euthyroid. These observations are of interest in relation to thyroid storm associated with bacterial infection.     

A comparison of the febrile responses of the Brattleboro and Sprague-Dawley strains of rats to endotoxin and endogenous pyrogens

The febrile responses of homozygous (di/di) Brattleboro rats, to both intravenous endogenous pyrogen and to a lipopolysaccharide endotoxin, were compared with those of normal Sprague-Dawley rats. There were no detectable differences between the fever curves of the two strains in response to endogenous pyrogen. Brattleboro rats, which are deficient in the neuropeptide arginine vasopressin (AVP), displayed fevers that were both qualitatively and quantitatively indistinguishable from those of normal Sprague-Dawley rats that do not suffer from congenital diabetes insipidus. It is concluded that the absence of AVP-containing cells in Brattleboro rats is not an important factor in determining the nature of their febrile responses to endogenous pyrogen. More remarkable, however, were the divergent febrile responses of the two strains to intravenously injected endotoxin. Normal rats displayed hypothermic responses, whereas the Brattleboro rats became febrile. By 2 h after the injection of endotoxin, body temperatures in both strains had returned to normal. Three hours after the rats had been exposed to endotoxin, both strains were found to be totally refractory to endogenous pyrogen. However, when both strains of rats were tested to endogenous pyrogen 3 days later, their febrile responses were more than double the magnitude of their initial control responses. These alterations in the febrile responsiveness of rats occurring at different times after the injection of endotoxin appear to be related to the effects that endotoxin has on the cells of the reticuloendothelial system, over the same time course.     

Increase in rat intestinal permeability to endotoxin during hyperthermia

Victims of heat stroke exhibit several clinical features which are also encountered in endotoxaemia. In order to investigate these similarities hyperthermic rats were used to explore the possibility that high body temperature results in increased permeability of intestinal wall to endotoxin. 125I endotoxin was introduced into intestinal segments taken from non-heat exposed rats. The segments were then incubated at 37 degrees C or 45 degrees C. Intestinal segments from heat stressed rats were similarly prepared and incubated at 37 degrees C. Leakage of endotoxin from segments taken from heat stressed rats was three times greater than from those from non-heat stressed rats, as were the segments from non-heat stressed rats which were incubated at 45 degrees C. These results indicate that the intestinal membrane is damaged by heat and that an increase in outward leakage of microbial endotoxins from the gut then occurs. This might contribute to the pathophysiological picture of heatstroke.     

Microwave radiation (2450 MHz) alters the endotoxin-induced hypothermic response of rats

The parenteral administration of bacterial endotoxin to rats causes a hypothermia that is maximal after approximately 90 minutes. When endotoxin-injected rats were held in a controlled environment at 22°C and 50% relative humidity and exposed for 90 minutes to microwaves (2450 MHz, CW) at 1 mW/cm², significant increases were observed in body temperature compared with endotoxintreated, sham-irradiated rats. The magnitude of the response was related to power density (10 mW/cm² > 5 mW/cm² > 1 mW/cm²). Saline-injected rats exposed for 90 minutes at 5 mW/cm² (specific absorption rate approximately 1.0 mW/g) showed no significant increase in body temperature compared with saline-injected, sham-irradiated rats. The hypothermia induced by endotoxin in rats was also found to be affected by ambient temperature alone. Increases in ambient temperature above 22°C in the absence of microwaves caused a concomitant increase in body temperature. This study reveals that subtle microwave heating is detectable in endotoxin-treated rats that have an impaired thermoregulatory capability. These results indicate that the interpretation of microwave-induced biological effects observed in animals at comparable rates and levels of energy absorption should include a consideration of the thermogenic potential of microwaves.