Fever: causes and consequences

Brain temperature fluctuation: a reflection of functional neural activation

Although it is known that relatively large increases in local brain temperature can occur during behaviour and in response to various novel, stressful and emotionally arousing environmental stimuli, the source of this heat is not clearly established. To clarify this issue, we monitored the temperature in three brain structures (dorsal and ventral striatum, cerebellum) and in arterial blood at the level of the abdominal aorta in freely moving rats exposed to several environmental challenges ranging from traditional stressors to simple sensory stimuli (cage change, tail pinch, exposure to another male rat, a female rat, a mouse or an unexpected sound). We found that brain temperature was consistently higher than arterial blood temperature, and that brain temperature increased prior to, and to a greater extent than, the increase in blood temperature evoked by each test challenge. Thus, the local metabolic consequences of widely correlated neural activity appear to be the primary source of increases in brain temperature and a driving force behind the associated changes in body temperature.

Long-term consequences of social stress on corticosterone and IL-1beta levels in endotoxin-challenged rats

Social stress has strong and long-lasting effects on autonomic nervous, neuroendocrine and behavioural functioning. The functionality of the immune system is profoundly influenced by autonomic nervous and neuroendocrine activities. Changes in sympathetic-adrenal and hypothalamic-pituitary-adrenocortical (HPA) axis activities as observed during and after social defeat, therefore, probably represent an important factor in the modulation of the immune response. In the present study, the impact of social defeat stress on the responsiveness of the immune system was studied by the presentation of a systemic inflammatory challenge through the injection of the bacterial endotoxin lipopolysaccharide (LPS). Male Wistar rats were subjected twice to social defeat 7 days apart. One week after the second defeat, they were injected with LPS in a low (150 microg/kg; D<LPS) or a high dose (375 microg/kg; D>LPS). Another group of defeated rats was injected with saline. Control, nondefeated rats also received the immune challenges. Previously defeated rats responded to the high dose of LPS with a deficient corticosterone (CORT) response resulting in an aggravated interleukin-1beta (IL-1beta) response 4 h after LPS injection. Furthermore, the experiments indicated that mortality rates after LPS administration were high in previously defeated rats, whereas mortality was absent in nondefeated rats. The results indicate that social stress has long-lasting effects on the functioning of the immune system and that it can seriously compromise the effectiveness of the adrenocortical response in containing some immunological defense mechanisms.

Some historical perspectives on thermoregulation

In this paper, selected historical aspects of thermoregulation and fever are presented as background to the application of molecular biology to thermoregulation. Temperature-sensing mechanisms, coordination of thermal information, thermoregulatory circuitry, efferent responses to thermal stimuli, set point mechanisms, and some of the mechanisms and consequences of fever and hyperthermia are highlighted. Neurotransmitters used in thermoregulatory circuits are also discussed. An attempt is made to include information from comparative physiological sources. Possible future avenues of research in the light of recent new technologies are also presented.

Impact of climate on thermal rhythm in pastoral sheep

The effects of environmental conditions on temperature rhythms were investigated in ewe lambs at pasture. Two groups of 20 lambs had heart rate (HR), vaginal temperature (T(v)), ear-canal temperature (T(c)) and ear-pinna temperature (T(p)) monitored continuously for 3 days. Climatic conditions were recorded at the same time and Temperature Humidity Index (THI) calculated. One group experienced fine clear weather for the 3 days, the other group experienced 2 days of heavy rain. During periods of fine weather, the daily rhythm for T(v) and T(c) was monophasic. However, heavy rain and a constant THI reduced the amplitude of the recorded temperature rhythms. Daily T(v) and T(c) patterns correlated strongly with THI, with a phase lag of 2 h. Peak T(v) and T(c) were at approximately 17:00 h each day. Mean maximum daily amplitudes were approximately 1.3 degrees C for T(v) and T(c). Mean T(v) was 39.3+/-0.1 and 39.6+/-0.1 for weeks 1 and 2, respectively, while mean T(c) was 38.9+/-0.1 and 39.2+/-0.1. Changes in T(v) and T(c) were closely correlated. We conclude that climate has a major effect on body temperature rhythms.

Prenatal exposure to nicotine attenuates stress-induced hyperthermia in 7- to 8-week-old rats upon exposure to a novel environment

Given that approximately 25% of women in the United States continue to smoke cigarettes during pregnancy, it is important to know if exposure to nicotine during development alters the physiological response of the adult to the "stressors" of everyday life. Our current experiments were carried out to determine if prenatal exposure to nicotine alters "stress-induced hyperthermia" in adult rats upon exposure to a novel environment such as a simulated open field. Forty-eight rats (23 males and 25 females) were exposed to a simulated open field or left in their home cage at 7 to 8 weeks of postnatal life (i.e., adulthood as defined by the ability to reproduce) following prenatal exposure to vehicle or nicotine (6 mg of nicotine tartrate per kilogram of maternal body weight per day) via a maternally implanted osmotic minipump from Day 6 or 7 of gestation. The simulated open field consisted of a 30(W)x60(L)x24(H)-in. white acrylic finish box illuminated by two hanging fluorescent lights and core temperature was measured by telemetry. Exposure to a simulated open field following prenatal exposure to vehicle elicited an increase in core temperature in male and female rats with a magnitude of approximately 1.2 degrees C and a duration of greater than 170 min. Prenatal exposure to nicotine significantly attenuated the thermogenic response of both genders; this was not only evident in the latency, magnitude and duration of the core temperature response but also in the core temperature index determined from the 3-h period following exposure to a simulated open field. Thus, our data provide evidence that prenatal exposure to nicotine attenuates stress-induced hyperthermia in male and female 7- to 8-week-old rats upon exposure to a "stressor" of everyday life (i.e., a novel environment).

Relationship between mean body surface temperature measured by use of infrared thermography and ambient temperature in clinically normal pigs and pigs inoculated with Actinobacillus pleuropneumoniae

OBJECTIVE

To determine the relationship between ambient temperature and mean body surface temperature (MBST) measured by use of infrared thermography (IRT) and to evaluate the ability of IRT to detect febrile responses in pigs following inoculation with Actinobacillus pleuropneumoniae.

ANIMALS

28 crossbred barrows.

PROCEDURES

Pigs (n = 4) were subjected to ambient temperatures ranging from 10 to 32 C in an environmental chamber. Infrared thermographs were obtained, and regression analysis was used to determine the relationship between ambient temperature and MBST. The remaining pigs were assigned to groups in an unbalanced randomized complete block design (6 A pleuropneumoniae-inoculated febrile pigs [increase in rectal temperature > or = 1.67 C], 6 A pleuropneumoniae-inoculated nonfebrile pigs [increase in rectal temperature < 1.67 C], and 12 noninoculated pigs). Infrared thermographs and rectal temperatures were obtained for the period from 2 hours before to 18 hours after inoculation, and results were analyzed by use of repeated-measures ANOVA.

RESULTS

A significant linear relationship was observed between ambient temperature and MBST (slope, 0.40 C). For inoculated febrile pigs, a treatment X method interaction was evident for rectal temperature and MBST, whereas inoculated nonfebrile pigs only had increased rectal temperatures, compared with noninoculated pigs. A method X time interaction resulted from the longer interval after inoculation until detection of an increase in MBST by use of IRT.

CONCLUSIONS AND CLINICAL RELEVANCE

Infrared thermography can be adjusted to account for ambient temperature and used to detect changes in MBST and radiant heat production attributable to a febrile response in pigs.

Vagotomy does not affect thermal responsiveness to intrabrain prostaglandin E2 and cholecystokinin octapeptide

Subdiaphragmatic vagotomy has been repeatedly shown to attenuate the febrile response to peripherally injected pyrogens. In the present study, we investigated whether vagotomy-induced attenuation of febrile responsiveness reflects a decreased sensitivity of the brain to central fever mediators, prostaglandin E2 (PGE2) and cholecystokinin octapeptide (CCK-8). Male rats were subjected to subdiaphragmatic vagotomy (or sham surgery) on day 0 and had a cannula implanted into the lateral cerebral ventricle on day 24. On day 30-36, the thermal responsiveness of the rats to PGE2 or CCK-8 was tested. Each animal was injected in the ventricle with either PGE2 (0, 10, 100, or 500 ng) in pyrogen-free saline with 0.5% ethanol (5 microl) or CCK-8 (0 or 1.6 microg) in artificial cerebro-spinal fluid (5 microl). While the 0-dose of either PGE2 or CCK-8 (vehicle alone) induced no thermal response, all the higher doses of either agent caused a body temperature rise preceded by tail skin vasoconstriction. The vagotomized rats did not respond differently than their sham-operated counterparts to any dose of either drug. It is concluded that subdiaphragmatic vagotomy does not change the rat's thermal responsiveness to intrabrain PGE(2) and CCK-8.

Suppression of PGE(2) fever at near term: reduced thermogenesis but not enhanced vasopressin antipyresis

Fevers are known to be suppressed near term in the mother, but the mechanism responsible for this phenomenon is not understood. We tested the hypothesis that the suppression of fever at term is a result of enhanced vasopressin-induced antipyresis. Effects of intracerebroventricular prostaglandin E(2) (PGE(2)) were examined in rats at gestational days 16-17 and 19-20 (near term) and days 1-2 postpartum. PGE(2) (50 ng) elevated body and interscapular brown adipose tissue (iBAT) temperatures and increased sympathetic nerve activity to the iBAT. PGE(2)-induced changes in iBAT temperature and nerve activity, as well as in rectal temperature, were reduced or eliminated near term, and responses were recovered in the postpartum period. Blood pressure and heart rate changes induced by central PGE(2) were also decreased at near term. Coinfusion of Manning compound, a V(1) vasopressin receptor antagonist, with PGE(2) throughout the peripartum period did not reverse the suppressed iBAT temperature and nerve activity or body temperature responses to PGE(2). Microdialysis experiments revealed unchanged terminal release of vasopressin in the ventral septal area after PGE(2) infusion in either pregnant or parturient rats. These results suggest that fever reduction at near term is not associated with enhanced vasopressin antipyresis, but may be a result of reduced sympathetic tone and in particular a reduced sympathetic drive to the iBAT. This finding may reflect a generalized reduction in autonomic output around the time of parturition.

Role of AVP in mediating the altered core temperature response to a simulated open field in pregnant rats

Near the term of pregnancy, rats have an attenuated core temperature response on exposure to a novel environment (e.g., a simulated open field) compared with that observed early in pregnancy or in nonpregnant rats. The present experiments were carried out on 26 nonpregnant and 26 pregnant rats to test the hypothesis that arginine vasopressin, functioning as an endogenous antipyretic substance in the central nervous system, mediates this attenuated core temperature response. Exposure to a simulated open field after intracerebroventricular (ICV) vehicle produced a significant increase in core temperature in both nonpregnant and pregnant animals, the magnitude and duration of which were greater in the nonpregnant rats. In nonpregnant rats, exposure to a simulated open field after ICV vasopressin V(1)-receptor antagonist altered the pattern of the core temperature response but not the core temperature index compared with that observed on exposure to a simulated open field after ICV vehicle. In pregnant animals, ICV vasopressin V(1)-receptor antagonist did not alter the core temperature response to a simulated open field compared with that observed after ICV vehicle. Thus our data do not support the hypothesis that a pregnancy-related activation of arginine vasopressin attenuates the core temperature response to a simulated open field in rats near the term of pregnancy.

Reduced corticotropin-releasing factor and enhanced vasopressin gene expression in brains of mice with autoimmunity-induced behavioral dysfunction

The spontaneous development of autoimmune disease in MRL-lpr mice induces behavioral and endocrine changes that resemble effects of chronic stressors. To further examine the correspondence between autoimmune disease and chronic stress, we asked whether the brains of autoimmune mice show a shift in the corticotropin-releasing factor (CRF) to vasopressin (AVP) ratio. Using in situ hybridization histochemistry with 35S-labelled mouse riboprobes, the levels of mRNA transcripts encoding CRF and AVP were compared between autoimmune MRL-lpr and control MRL +/+ brains. CRF transcript levels were lower in the hypothalamic paraventricular nucleus and in the central nucleus of the amygdala in MRL-lpr mice. AVP transcript levels were higher in the paraventricular and the supraoptic nuclei in MRL-lpr mice compared to controls. CRF mRNA levels were inversely related to performance in stress-sensitive tasks and to measures of autoimmunity. As found previously for behavioral performance, immunosuppressive treatment with cyclophosphamide abolished the group difference in neuropeptide gene expression. These results indicate that an autoimmune disease process is necessary for the shift in the brain CRF:AVP ratio. Furthermore, they support the parallel between chronic stress and chronic autoimmunity/inflammation, and suggest common central mechanisms relevant to endocrine function and behavior.

Arginine vasopressin, fever and temperature regulation

While central administration of arginine vasopressin (VP) to the non-febrile rat at high doses can cause hypothermia, there is little evidence for a role for endogenous VP in normal thermoregulation. In contrast, VP arising from cell bodies in the bed nucleus of the stria terminalis and innervating the ventral septal areas and possibly the amygdala appears to be an endogenous antipyretic, i.e. a substance capable of reducing fever. As the synthesis of VP in bed nucleus neurons is dependent upon circulating androgens, female rats have much less VP in these cells and their projections than do male rats. In keeping with this, females may make use of VP to a lesser extent than do males to bring about antipyresis. The phenomenon whereby the VP receptor can become sensitized by previous exposure to VP may be responsible for some states of endogenous antipyresis, in which fevers are suppressed through overactivity of the vasopressinergic system. States of endogenous antipyresis can be revealed around the time of parturition in both the neonate and the mother.

Sympathoadrenal hyperactivity and the etiology of neuroleptic malignant syndrome

OBJECTIVE

The author's goal was to develop a pathophysiological model for neuroleptic malignant syndrome with greater explanatory power than the alternative hypotheses of hypothalamic dopamine antagonism (elevated set point) and direct myotoxicity (malignant hyperthermia variant).

METHOD

Published clinical findings on neuroleptic malignant syndrome were integrated with data from human and animal studies of muscle physiology, thermoregulation, and autonomic nervous system function.

RESULTS

The data show that the sympathetic nervous system's latent capacity for autonomous activity is expressed when tonic inhibitory inputs from higher central nervous system centers are disrupted. These tonic inhibitory inputs are relayed to preganglionic sympathetic neurons by way of dopaminergic hypothalamospinal tracts. The sympathetic nervous system mediates hypothalamic coordination of thermoregulatory activity and is a primary regulator of muscle tone and thermogenesis, augmenting both of these when stimulated. In addition, the sympathetic nervous system modulates all of the other end-organs that function abnormally in neuroleptic malignant syndrome.

CONCLUSIONS

There is substantial evidence to support the hypothesis that dysregulated sympathetic nervous system hyperactivity is responsible for most, if not all, features of neuroleptic malignant syndrome. A predisposition to more extreme sympathetic nervous system activation and/or dysfunction in response to emotional or psychological stress may constitute a trait vulnerability for neuroleptic malignant syndrome, which, when coupled with state variables such as acute psychic distress or dopamine receptor antagonism, produces the clinical syndrome of neuroleptic malignant syndrome. This hypothesis provides a more comprehensive explanation for existing clinical data than do the current alternatives.

Gender influences the core temperature response to a simulated open field in adult guinea pigs

The induction of psychological stress is often accompanied by a transient increase in core temperature, commonly referred to as stress induced hyperthermia. Although stress-induced hyperthermia occurs when rats, mice, and pigs are exposed to a novel stimulus (e.g., a simulated open field, restraint, etc.), whether or not it occurs in guinea pigs has not been investigated. The present experiments were therefore carried out to investigate the thermoregulatory responses of both male (n = 7) and female (n = 7) adult guinea pigs when they were exposed to a simulated open field. Unexpectedly, neither the male nor female guinea pigs developed stress-induced hyperthermia. To the contrary, female but not male guinea pigs significantly decreased their core temperature during an open field experiment. The mechanism of the gender-specific thermoregulatory response of the adult guinea pig to psychological stress is presently unknown.

Tumor cell apoptosis, lymphocyte recruitment and tumor vascular changes are induced by low temperature, long duration (fever-like) whole body hyperthermia

A single treatment of low-temperature, long-duration, whole-body hyperthermia of either severe combined immunodeficient (SCID) mice bearing human breast tumor xenografts or Balb/c mice bearing syngeneic tumors for 6-8 hr can cause a temporary reduction of tumor volume and/or a growth delay. In both animal model systems, this inhibition is correlated with the appearance of large numbers of apoptotic tumor cells. Because this type of mild heat exposure, comparable to a common fever, is not itself directly cytotoxic, other explanations for the observed tumor cell death were considered. Our data support the hypothesis that this hyperthermia protocol stimulates some component(s) of the immune response, which results in increased antitumor activity. In support of this hypothesis, increased numbers of lymphocyte-like cells, macrophages, and granulocytes are observed in the tumor vasculature and in the tumor stroma immediately following this mild hyperthermia exposure. In Balb/c mice, an infiltrate persists in the tumor for at least 2 weeks. Using the SCID mouse/human tumor system, we found that both host natural killer (NK) cells and injected human NK cells were increased at the site of tumor following hyperthermia treatment. Experiments using anti-asialo-GM1 antibodies indicate that the tumor cell apoptosis seen in the SCID mouse appears to be due largely to the activity of NK cells, although additional roles for other immunoeffector cells and cytokines appear likely in the immunologically complete Balb/c model. Another interrelated hypothesis is that immunoeffector cells may have greater access to the interior of the tumor because we have observed that this treatment causes an obvious expansion in the diameter of blood vessels within the tumor and an increase in nucleated blood cells within the vessels, which persists as long as 2 weeks after treatment. Further study of the mechanisms by which mild hyperthermia exerts antitumor activity could result in this treatment protocol being used as an effective, nontoxic adjuvant to immunotherapy and/or other cancer therapies.

AVP mediates the attenuated febrile response to administration of PGE1 in rats near term of pregnancy

Rats have an attenuated febrile response to intracerebroventricular injection of PGE1 near the term of pregnancy, the mechanism of which is unknown. The present experiments were carried out to test the hypothesis that arginine vasopressin (AVP), functioning as an endogenous antipyretic substance in the central nervous system, mediates this attenuated febrile response. The febrile response to intracerebroventricular injection of 0.2 microg PGE1 was determined in pregnant and nonpregnant rats after an intracerebroventricular injection of either vehicle or a vasopressin V1-receptor antagonist. After intracerebroventricular administration of vehicle, intracerebroventricular administration of 0.2 microg PGE1 produced significant increases in core temperature in both nonpregnant and pregnant animals. The increase in core temperature, however, was attenuated both in magnitude and duration in pregnant compared with nonpregnant animals. After intracerebroventricular administration of a vasopressin V1-receptor antagonist, intracerebroventricular administration of 0.2 microg PGE1 produced significant increases in core temperature that were similar in nonpregnant and pregnant animals. Our data support the hypothesis that a pregnancy-related activation of AVP as an endogenous antipyretic substance in the central nervous system attenuates the febrile response to intracerebroventricular administration of PGE1 near term of pregnancy in rats.

Fever after craniofacial surgery in the infant under 24 months of age

A retrospective review was undertaken of 126 consecutive craniofacial procedures involving a transcranial component, performed at the Children's Medical Center at Dallas, between 1990 and 1994. Standard postoperative axillary temperature measurements were recorded until discharge. Age at surgery of less than 24 months correlated very strongly with a postoperative temperature of greater than 38 degrees C (r = -0.92). The incidence of postoperative fever was high in all age groups, yet there was still a significant difference between the group younger than 2 years and the group in which surgery was performed after the age of 2 years across all postoperative temperature ranges, from >38 degrees C to >39.5 degrees C (p < 0.001, chi-square test). The white blood cell count was elevated above the age-related normal in 67 percent of febrile patients. There was no correlation between type or duration of surgical procedure, length of intensive care or hospital stay, or the need for blood transfusion and the development of a significant postoperative fever. There were minor infectious complications in four patients (3 percent), only one of which was a wound problem related to the surgery. All infectious complications were easily identifiable clinically. There was no mortality or serious infections. The development of postoperative fever, and an elevated white blood cell count, is to be expected in pediatric patients undergoing craniofacial procedures. The routine laboratory investigation of postoperative fever in pediatric craniofacial patients under 2 years of age without procedures involving transgression of the paranasal sinuses is not warranted unless there are associated clinical indicators.

Methodology of fever research: why are polyphasic fevers often thought to be biphasic?

This study explains why the recently described triphasic lipopolysaccharide (LPS) fevers have been repeatedly mistaken for biphasic fevers. Experiments were performed in loosely restrained male Wistar rats with a catheter implanted into the right jugular vein. Each animal was injected with Escherichia coli LPS, and its colonic (Tc) and tail skin temperatures were monitored. The results are presented as time graphs and phase-plane plots; in the latter case the rate of change of Tc is plotted against Tc. At an ambient temperature (Ta) of 30.0 degrees C, the response to the 10 microg/kg dose of LPS was triphasic, as is obvious from time graphs of Tc (3 peaks), time graphs of effector activity (3 waves of tail skin vasoconstriction), and phase-plane plots (3 complete loops). When the Ta was below neutral (22.0 degrees C) or the LPS dose was higher (100 or 1,000 microg/kg), the time graph of Tc did not allow for the reliable detection of all three febrile phases, but the phase-plane plot and time graph of effector activity clearly revealed the triphasic pattern. In a separate experiment, LPS (10 microg/kg) or saline was injected via one of two different procedures: in the first group the injection was performed through the jugular catheter, from outside the experimental chamber; in the second group the same nonstressing injection was combined with opening the chamber and pricking the animal in its lower abdomen with a needle. In the first group the febrile response was obviously triphasic, and none of the phases was due to the procedure of injection per se (injection of saline did not affect Tc). In the second group the fever similarly consisted of three Tc rises, but it might have been readily mistaken for biphasic because the first rise was indistinguishable from stress hyperthermia occurring in the saline-injected (and needle-pricked) controls. We conclude that several methodological factors (dose of LPS, procedure of its injection, and Ta) have contributed, although each in a different way, to the common misbelief that there are only two febrile phases.

Neuroglia: possible role in thermogenesis and body temperature control

The neuroglia, especially astrocytes, constitute a cell mass capable of adaptive heat production, since both the metabolic substrates and the biochemical machinery for energy production and its regulation seem to be available in these cells. Earlier physiological studies from this laboratory have provided circumstantial evidence that rodents such as rats and rabbits may indeed be capable of increasing their cerebral heat production during acute cold exposure. Recent relevant literature on the ability of neuroglia of the mammalian CNS to synthesize and release different transmitters and modulators and to communicate mutually with neuronal elements is discussed in support of the idea that different glial cell types could also contribute to the central regulation of body temperature in addition to the more established similar function of the neuronal pathways. The present hypothesis may have relevance to changes in glial cell mass and activity that occur in patients during the course of aging, or in gliosis with a consequent tendency for epilepsy caused by head trauma, with a consequent decrease or increase of intracranial metabolic rate, respectively. Also, the possibility for glial contribution to the thermoregulatory changes seen in psychoses is discussed.

Pyretic and antipyretic signals within and without fever: a possible interplay

Current concepts on the pathogenesis of fever emphasize the importance of the cytokine-prostaglandin cascade. This humoral line mediates nonthermal signals to the brain, while the thermal signals supply feedback from the thermoreceptors. However, the humoral line cannot alone account for the whole febrile response. Here, we hypothesize that, besides this humoral mediatory mechanism, nonthermal neural signals of abdominal origin conveyed mainly by the vagus nerve are also important pro-pyretic factors. The pro-pyretic mechanisms are proposed to be in a dynamic balance with endogenous antipyretic mechanisms that also form an integral part of the normal reaction to pyrogens. Further, it is hypothesized that the role of such neural and humoral signals either for elevation or depression of body temperature is not limited to fever but has an important role also in nonfebrile thermoregulation.

Intracerebroventricular prostaglandin administration increases the neural damage evoked by global hemispheric hypoxic ischemia

This study was designed to determine if central (intracerebroventricular, i.c.v.) administration of prostaglandin E2 (PGE2, mediator of core temperature elevation following exogenous or endogenous pyrogen administration) worsens the neural damage of anesthetized rats to global hemispheric hypoxic-ischemia (GHHI) from damage seen in normothermic, i.c.v. saline control groups. The first study (no GHHI) showed that 10 or 50 ng PGE2 given i.c.v. to groups of anesthetized Long-Evans rats evoked dose-related increases in colonic (systemic core) temperature but no neural damage. In the second study anesthetized rats were given an i.c.v. injection of sterile saline or PGE2 plus GHHI (ligation of the right common carotid artery plus 35 min of 12% O2) at the peak of the temperature response. Thermal response indices (TRI, degrees C x min), determined from brain (temporalis muscle, ipsilateral and contralateral to ligation) and core (colonic) temperatures, showed significant increases in the 50-ng PGE2 group compared to the TRIs of the 10-ng PGE2 or saline control group. The 50-ng PGE2, GHHI group had a higher mortality rate and showed greater ipsilateral hemispheric neural damage than the saline-treated group given the same insult, especially due to increased damage to the cortex. The results show that i.c.v. PGE2 administration significantly increases the neural damage caused by GHHI, possibly due to the associated rise in core temperature.

Regulation of prostaglandin biosynthesis in vivo by glutathione

Intraperitoneal administration of urate crystals to mice reduced subsequent macrophage conversion of arachidonic acid (AA) to prostaglandins (PGs) and 12-hydroxyeicosatetraenoic acid for up to 6 h. In contrast, levels of 12-hydroxyheptadecatrienoic acid (12-HHT) were markedly elevated. This metabolic profile was previously observed in vitro when recombinant cyclooxygenase (COX) enzymes were incubated with reduced glutathione (GSH). Analysis of peritoneal GSH levels revealed a fivefold elevation after urate crystal administration. The GSH synthesis inhibitor L-buthionine-[S,R]-sulfoximine partially reversed the urate crystal effect on both GSH elevation and PG synthesis. Moreover, addition of exogenous GSH to isolated peritoneal macrophages shifted AA metabolism from PGs to 12-HHT. Urate crystal administration reduced COX-1, but induced COX-2 expression in peritoneal cells. The reduction of COX-1 may contribute to the attenuation of PG synthesis after 1 and 2 h, but PG synthesis remained inhibited up to 6 h, when COX-2 levels were high. Overall, our results indicate that elevated GSH levels inhibit PG production in this model and provide in vivo evidence for the role of GSH in the regulation of PG biosynthesis.

Extracts of rhinoceros horn are not antipyretic in rabbits

We administered extracts of the horn of the African Black rhinoceros intragastrically to 7 rabbits, at the same time as injecting bacterial lipopolysaccharide (LPS) intravenously into the rabbits to produce fever. At a dose of horn (50 mg/kg) similar to that allegedly used to reduce human fever, and at ten times that dose, the fever response to LPS was not significantly different (P > 0.05, t-test) to the response to LPS injection when boiled water was administered instead of horn extract. The known antipyretic indomethacin, however, at a dose of 10 mg/kg significantly reduced the response to LPS.

The seasonal distribution of infant deaths by age: a comparison of sudden infant death syndrome and other causes of death

OBJECTIVE

To examine the possibility that among deaths in infancy the increase in the winter/summer ratio with increasing age is not peculiar to sudden infant death syndrome (SIDS).

METHODOLOGY

Details of the winter (December-February)/summer (June-August) ratio among deaths in neonates (< 28 days) and post neonates dying in the United States of America between 1979 and 1990 were abstracted from published statistics. The primary causes of death were classified according to the ninth Revision of the International Classification of Diseases.

RESULTS

For every non-traumatic cause of death including SIDS, the winter/summer ratio was higher among postneonates than neonates. This was not seen for deaths due to trauma. Cases of SIDS and deaths due to infection had the highest ratios in both age categories. Causes of death occurring predominantly in the neonatal period (e.g. anencephaly) had the lowest overall ratios.

CONCLUSIONS

Neither the greater number of SIDS cases in the winter, nor the increasing winter/summer ratio with increasing age is unique to SIDS.

Reduced preference for sucrose in autoimmune mice: a possible role of interleukin-6

In a continuous one-bottle sucrose intake test, 4-month-old autoimmune MRL-Ipr mice show a shift to the right along the X-axis of the concentration-intake function, compared to congenic MRL +/+ controls. Using a brief (60-min) and a continuous (48-h) two-bottle test, the present report examines potential factors that could account for the reduced responsiveness to a palatable stimulus. Study 1 examines whether preference for sucrose is associated with age, changes in food/water intake, or impaired renal function. Reduced preference for sucrose was observed in 5-6-week-old MRL-Ipr males, although food/water intake or blood creatinine levels did not differ from control values. Immunosuppressive treatment abolished this deficit, suggesting a role of immune factor(s). Study 2 tests the hypothesis that chronic upregulation of the neuroactive cytokine interleukin-6 (IL-6), reported to occur from 3 weeks of age in young MRL-Ipr mice, reduces preference for sucrose. Sustained administration of IL-6 was produced by infecting healthy MRL +/+, C3H.SW and Balb/C mice with adenovirus vector carrying cDNA for murine IL-6. This resulted in high serum IL-6 levels over 5 days, a rapid decline in preference for sucrose and low blood glucose levels. The results from Study 1 indicate that impaired sensitivity to sucrose in MRL-Ipr mice can be detected before autoimmune disease is florid in MRL-Ipr mice. The results from Study 2 are consistent with altered motivation/emotional states after infection, and point to sustained IL-6 production as an early mechanism in behavioral alterations during chronic autoimmune/inflammatory conditions.

Incidence of fever in the rehabilitation phase following brain injury

There appears to be a high incidence of fever after brain injury. The most common cause for fever is infection. The incidence of fever occurring as a result of hypothalamic thermoregulatory dysfunction after brain injury is less clear. This study retrospectively reviewed the charts of 286 subjects with brain injuries. Subject subpopulations were divided into traumatic brain injuries, anoxic brain injuries, and brain injuries resulting from aneurysmal subarachnoid hemorrhage. Fever events were described as any core temperature greater than 99.9 degrees F. Most subjects suffered a severe brain injury and had an average acute hospital length of stay ranging from 35.4 to 60 days. The average rehabilitation length of stay ranged from 38.4 to 45.1 days. Twenty-four percent of subjects experienced fevers, with each of the populations having similar occurrence rates. Unexplained fever events were found in the traumatic brain injury (7%) and aneurysmal subarachnoid hemorrhage (8%) subpopulations only. No unexplained fever event was associated with a temperature greater than 100.8 degrees F.

Suprachiasmatic nuclei lesions do not eliminate homeostatic thermoregulatory responses in rats

Electrolytic lesions aimed at the suprachiasmatic nuclei (SCN) were made in male Long-Evans rats. Body temperature (Tb), activity, and drinking were monitored continuously in a 12-h light:12-h dark (12:12 LD) cycle at an ambient temperature of 23 degrees C. Large SCN lesions eliminated activity and drinking rhythms and abolished or reduced the circadian rhythm of Tb. The Tb responses of the rats were measured in L after exposure to cold and injection of lipopolysaccharide (LPS), a fever-producing drug, and in both L and D during a 30-min exposure to a novel cage. Rats with SCN lesions (SCNX) maintained their Tb as well as did controls during 2-h exposure to 2 degrees C. They also showed the expected increases in Tb in response to novelty and LPS. Nevertheless, there were differences between SCNX rats and other rats. When measured 9 h after LPS injection, SCNX rats had lower Tb in D than did sham-lesioned or intact rats or rats with lesions that missed the SCN. This is not surprising; the Tb of SCNX rats does not go as high as that of intact rats in D. However, it was surprising that at night SCNX rats increased their Tb in response to novelty (lights on in the test situation), whereas normal rats did not. For some reason, light inhibits the Tb rise to novelty in normal rats but does not do so in rats with SCN lesions.

Defect in interleukin-1beta secretion prevents sickness behavior in C3H/HeJ mice

To examine the role of interleukin-1beta (IL-1beta) in mediating sickness, we studied the effects of lipopolysaccharide (LPS) and IL-1beta on social behavior in endotoxin-responsive C3H/HeOuJ (OuJ) mice and endotoxin-resistant C3H/HeJ (HeJ) mice. Whereas LPS (1, 10 and 100 microg) depressed social behavior and body weight compared to saline in OuJ mice, in HeJ mice it did not. To determine if the refractoriness of HeJ mice to the behavioral effects of LPS was related to secretion of IL-1beta, in a second study, HeJ and OuJ mice were injected IP with LPS (10 microg) and plasma concentration of IL-1beta was determined postinjection. At 4 h postinjection, the plasma concentration of IL-1beta was increased by LPS in OuJ mice, but not in HeJ mice. The increase in plasma IL-1beta in OuJ mice corresponded to the maximal depression in social behavior. To further verify that HeJ mice are refractory to the behavioral effects of LPS because they fail to respond and produce cytokines, the social behavior of HeJ and OuJ mice injected IP with recombinant murine IL-1beta (0, 50, 100, or 200 ng) was compared. As anticipated, exogenous IL-1beta depressed social behavior similarly in endotoxin-responsive OuJ mice and endotoxin-resistant HeJ mice. These data indicate that a genetic mutation in HeJ mice that prevents LPS-induced synthesis of cytokines also renders HeJ mice refractory to the behavioral effects of LPS.

Influence of pregnancy on the febrile response to ICV administration of PGE1 in rats studied in a thermocline

Rats near term of pregnancy have an attenuated febrile response to intracerebroventricular (ICV) injection of prostaglandin E1 (PGE1) when they are studied at an ambient temperature below their thermoneutral zone. Given that nonshivering thermogenesis in brown adipose tissue is impaired in rodents near term of pregnancy, it is possible that the attenuated febrile response is forced by impairment of this component of the autonomic thermoregulatory response. If this were the case, then near-term pregnant rats should develop a "normal" fever after PGE1 administration if they were studied in a thermocline where they could utilize behavioral as well as autonomic thermoregulatory effectors to increase their body core temperature (Tbc). Experiments were, therefore, carried out on 13 nonpregnant and 14 pregnant chronically instrumented rats in a thermocline (temperature gradient 10-40 degrees C) to investigate their Tbc responses to ICV injection of PGE1. ICV injection of 0.2 microgram PGE1 produced significant increases in Tbc and fever index in both nonpregnant and pregnant animals (day 19 of gestation); the increases, however, were significantly attenuated in the pregnant compared with the nonpregnant rats. Behavioral (e.g., selected ambient temperature) and autonomic (e.g., oxygen consumption) thermoregulatory effectors were activated to increase Tbc after ICV PGE1 in both groups of animals, but the duration of activation was shortened in pregnant compared with nonpregnant rats. The abbreviated thermoregulatory effector responses and the resulting attenuated febrile response to PGE1 in the pregnant rats may have resulted from a pregnancy-related activation of an endogenous antipyretic system.

Pregnancy alters body-core temperature response to a simulated open field in rats

Exposure of a rat to a novel environment (e.g., a simulated open field) induces a transient increase in body-core temperature, which is often called stress-induced hyperthermia. Although pregnancy is known to influence thermoregulatory control, its effect on stress-induced hyperthermia is unknown. Therefore, 24 Sprague-Dawley rats (8 nonpregnant and 16 pregnant) were studied to test the hypothesis that pregnancy would alter the development of stress-induced hyperthermia after exposure to a simulated open field. Body-core temperature index increased significantly after exposure to a simulated open field in nonpregnant and gestation day-10 rats but not in gestation day-15 and day-20 rats. Thus our data provide evidence that pregnancy influences the body-core temperature response of rats exposed to a simulated open field in a gestation-dependent fashion. The functional consequences as well as the mechanisms involved remain to be determined.

Hormones, lymphohemopoietic cytokines and the neuroimmune axis

The classical distinction between hormones and cytokines has become increasingly obscure with the realization that homeostatic responses to infection involve coordinated changes in both the neuroendocrine and immune systems. The hypothesis that these systems communicate with one another is supported by the ever-accruing demonstrations of a shared molecular network of ligands and receptors. For instance, leukocytes express receptors for hormones and these receptors modulate diverse biological activities such as the growth, differentiation and effector functions. Leukocyte lineages also synthesize and secrete hormones, such as insulin-like growth factor-I (IGF-I), in response to both growth hormone (GH) and also to cytokines such as tumor necrosis factor-alpha (TNF-alpha). Since hormones share intracellular signaling substrates and biological activities with classical lymphohemopoietic cytokines, neuroendocrine and immune tissues share a common molecular language. The physiological significance of this shared molecular framework is that these homeostatic systems can intercommunicate. One important example of this interaction is the mechanism by which bacterial lipopolysaccharide, by eliciting a pro-inflammatory cytokine cascade from activated leukocytes, modulate pituitary GH secretion as well as other CNS-controlled behavioral and metabolic events. This article reviews the cellular and molecular basis for this communication system and proposes novel mechanisms by which neuroendocrine-immune interactions converge to modulate disease resistance, metabolism and growth.

The afferent discharges from sensors for interleukin 1 beta in the hepatoportal system in the anesthetized rat

The effect of intraportal administrations of interleukin 1 beta (IL-1 beta) on the afferent activity of the hepatic branch of the vagus nerve was observed in urethane anesthetized rats. An intraportal injection of IL-1 beta in doses of 10 pg and 100 pg per animal (300-400 g body wt.) resulted in dose-dependent increase in the afferent activity, which lasted about 70-100 min. Further, intraportal injection of IL-1 beta (100 pg) induced reflex activation of efferent activity of the splenic (sympathetic) nerve and vagal thymic nerve. This reflex activation was not observed in hepatic vagotomized rat and after an administration of the same dose of IL-1 beta into the systemic vein in normal rat. The results suggest the existence of sensors for IL-1 beta which send information on IL-1 beta in the portal venous blood to the central nervous system through the hepatic branch of the vagus nerve and play some role in reflex regulation of immune function.

Renin stimulates decidual prostaglandin production via a novel mechanism that is independent of angiotensin II formation

Renin is a proteolytic enzyme that has been considered to have only one function which is to cleave angiotensinogen between the 10th and 11th amino acids to form angiotensin-1. This is then converted to angiotensin-II, a potent vasoconstrictor, antinatriuretic and antidiuretic by angiotensin-converting enzyme. We have investigated the action of renin to stimulate prostaglandin production by decidual cells and in so doing have generated data that challenge the prevailing dogma. Renin stimulates decidual prostaglandin production in a concentration-related fashion that is unaffected by saralasin treatment. This stimulatory action of renin is enhanced rather than reduced by arachidonic acid treatment but abolished by treatment with cycloheximide or actinomycin D. Renin caused a more rapid recovery of decidual prostaglandin biosynthesis from acetylsalicylic acid treatment than did control media. Moreover, renin treatment of both decidual and amnion cells induced increased levels of PGHS-2 within 2 h. Collectively, these results indicate that renin can act directly, separately from the generation of angiotensin-I and II. In this case renin can induce PGHS expression.

Neoplastic fever

Neoplastic fever, that is fever arising solely as a manifestation of malignancy, is not as common as was thought but still constitutes a troublesome symptom and is difficult to manage. The mechanism of neoplastic fever production involves cytokines such as tumour necrosis factor (TNF), interleukins 1 and 6 (IL-1, IL-6) and interferon (IFN), produced either by host macrophages in response to tumour, or sometimes by the tumour itself. The cytokines stimulate production of prostaglandins which act on the hypothalamus causing a change in the thermostatic set point. This mechanism is similar to that of infective fever. Paracetamol (acetaminophen) will often only partially lyse neoplastic fever but nonsteroidal anti-inflammatory drugs have been shown to be effective. On the basis of small studies, naproxen has been proposed as a useful test to discriminate between neoplastic and infective fever. If this is so, it must be acting through a pathway hitherto undescribed and specific to neoplastic fever. Other work shows that this group of drugs effectively lyses both types of fever. Therefore larger studies are needed to confirm or refute the "Naproxen test' and neoplastic fever remains a diagnosis of exclusion. In the future, cytokine antagonists may have a role in managing neoplastic fever, but, until their actions are better understood, nonsteroidal anti-inflammatory drugs remain the medication of choice if standard antipyretics have failed.

Alcohol stimulates ACTH secretion in the rat: mechanisms of action and interactions with other stimuli

This review discusses some of the mechanisms through which alcohol (EtOH) alters the activity of the hypothalamic-pituitary-adrenal (HPA) axis. In adult rats, acute EtOH treatment increases plasma ACTH and corticosteroids levels primarily by stimulating the release of corticotropin-releasing factor (CRF) and possibly vasopressin (VP) from nerve terminals in the median eminence. Increased CRF gene transcription in the hypothalamus may also be important. The HPA axis remains activated during chronic EtOH exposure, although habituation may take place. Changes in the responsiveness of hypothalamic neurons, a phenomenon itself dependent in part on a number of intermediate secretagogues, as well as decreased pituitary responsiveness to VP, all play a role. Finally, the activity of the HPA axis is influenced by exposure to EtOH during embryonic development, with mature offspring showing hyporesponsiveness to many stimuli. These altered responses appear to be caused in part by changes in the synthesis/release CRF, possibly under the influence of nitric oxide. CRF, VP, ACTH, and corticosteroids are important regulators of the immune system, behavior, metabolic pathways, and reproductive parameters. Alcohol therefore may influence such functions through the pathological secretion of these hormones. A better understanding of the mechanisms through which the drug alters their release thus may permit the development of therapies designed to alleviate some of the consequences of alcoholism.

Immune activation: the role of pro-inflammatory cytokines in inflammation, illness responses and pathological pain states

It has recently become accepted that the activated immune system communicates to brain via release of pro-inflammatory cytokines. This review examines the possibility that pro-inflammatory cytokines (interleukins and/or tumor necrosis factor) mediate a variety of commonly studied hyperalgesic states. We will first briefly review basic immune responses and inflammation. We will then develop the concept of illness responses and provide evidence for their existence and for the dramatic changes in neural functioning that they cause. Lastly, we will examine the potential roles that both pro-inflammatory cytokines and the neural circuits that they activate may play in the hyperalgesic states produced by irritants, inflammatory agents, and nerve damage. The possibility is raised that apparently diverse hyperalgesic states may converge in the central nervous system and activate similar or identical neural circuitry.

Cytokine-to-brain communication: a review & analysis of alternative mechanisms

It is becoming well accepted that products of the immune system (cytokines) can signal the brain that infection has occurred. This cytokine-to-brain communication can result in marked alterations in brain function and behavior. This review examines alternative mechanisms that have been proposed to explain how such immune products can reach the brain via the blood to cause centrally-mediated "illness" responses. Finally, we describe a new view which argues that cytokines signal brain in quite a different manner, by stimulating afferent terminals of peripheral nerves at local sites of synthesis and release.

Blockade of cytokine induced conditioned taste aversion by subdiaphragmatic vagotomy: further evidence for vagal mediation of immune-brain communication

Interleukin-1 beta (IL-1 beta) and tumor necrosis factor-alpha (TNF-alpha) are cytokines released by activated immune cells. IL-1 beta and TNF-alpha elicit various illness symptoms including avoidance of novel tastes with which they have been paired (conditioned taste aversion). Previous hypotheses to account for these actions have focused on blood-borne IL-1 beta and TNF-alpha exerting their effects directly at the brain. However, recent evidence suggests that these cytokines may activate subdiaphragmatic vagal afferents. The present experiments demonstrate that subdiaphragmatic vagal transection both attenuates acquisition and facilitates extinction of conditioned taste aversions induced by i.p. administration of either IL-1 beta or TNF-alpha.

Blockade of interleukin-1 induced hyperthermia by subdiaphragmatic vagotomy: evidence for vagal mediation of immune-brain communication

Interleukin-1 beta (IL-1 beta), a cytokine released by activated immune cells, elicits various illness symptoms including hyperthermia. Previous hypotheses to account for these actions have focused on blood-borne IL-1 beta exerting its effects directly at the level of the brain. However, recent behavioral and physiological evidence suggest that IL-1 beta can activate the subdiaphragmatic vagus. The present experiments demonstrate that subdiaphragmatic vagal transection disrupts the hyperthermia-inducing effects of recombinant human IL-1 beta and stress. These data provide evidence for a novel route of immune-brain communication, as well as a novel route whereby stress can influence physiological processes.