Helen P. Laburn

Interleukin-6 and leptin mediate lipopolysaccharide-induced fever and sickness behavior.

Pro-inflammatory cytokines, interleukin (IL)-1beta, IL-6 and tumor necrosis factor-alpha (TNF-alpha) synthesized by activated macrophages and monocytes in response to administration of lipopolysaccharide (LPS), are considered important mediators of fever and sickness behavior. We administered rat-specific antisera for TNF-alpha, IL-1beta, IL-6 and leptin, to determine the involvement of peripherally released cytokines in LPS-induced fever and sickness behavior, measured as suppression of voluntary wheel-running and food intake. Male Sprague-Dawley rats (approximately 200 g) selected for their predisposition to spontaneously run on running wheels were anaesthetized with a combination of ketamine hydrochloride (80 mg/kg i.m.) and xylazine (4 mg/kg i.m.) and implanted intra-abdominally with temperature-sensitive radiotelemeters. Rats were injected intraperitoneally with anti-rat sera to one of the following, TNF-alpha, IL-1beta, IL-6 or leptin or with pre-immune sheep serum, followed by a subcutaneous injection of either LPS (250 microg/kg) or sterile saline. Lipopolysaccharide administration induced a approximately 1.3 (0.2) degrees C fever lasting approximately 10 h and reduced voluntary running by 93 (8.6)% and food intake by 51 (21.3)% compared to the saline response (ANOVA, P<0.05). Injection of anti-IL-6 serum or anti-leptin serum abolished the LPS-induced fever, anti-TNF-alpha serum affected only the early phase of fever and anti-IL-1beta serum had no effect on fever (ANOVA, P<0.05). LPS-induced suppression of voluntary running and food intake were attenuated in rats receiving anti-IL-6 serum, while the decrease in food intake was totally abolished in rats receiving anti-leptin serum (ANOVA, P<0.05). Injection of anti-TNF-alpha or anti-IL-1beta serum had no effect on LPS-induced sickness behavior. Peripherally released IL-6 and leptin therefore appear to be important in regulating LPS-induced fever and sickness behavior.

Adaptive heterothermy and selective brain cooling in arid-zone mammals.

Adaptive heterothermy and selective brain cooling are regarded as important thermal adaptations of large arid-zone mammals. Adaptive heterothermy, a process which reduces evaporation by storing body heat, ought to be enhanced by ambient heat load and by water deficit, but most mammals studied fail to show at least one of those attributes. Selective brain cooling, the reduction of brain temperature below arterial blood temperature, is most evident in artiodactyls, which possess a carotid rete, and traditionally has been considered to protect the brain during hyperthermia. The development of miniature ambulatory data loggers for recording body temperature allows the temperatures of free-living wild mammals to be measured in their natural habitats. All the African ungulates studied so far, in their natural habitats, do not exhibit adaptive heterothermy. They have low-amplitude nychthemeral rhythms of temperature, with mean body temperature over the night exceeding that over the day. Those with carotid retes (black wildebeest, springbok, eland) employ selective brain cooling but zebra, without a rete, do not. None of the rete ungulates, however, seems to employ selective brain cooling to prevent the brain overheating during exertional hyperthermia. Rather, they use it at rest, under moderate heat load, we believe in order to switch body heat loss from evaporative to non-evaporative routes.

Interleukin (IL)-6 and IL-1β act synergistically within the brain to induce sickness behavior and fever in rats

Pro-inflammatory cytokines interleukin (IL)-6 and IL-1 beta can act in the brain (centrally) to cause fever. Sickness behaviors which accompany fever also appear to involve the central action of IL-1 beta. We injected species-homologous rat IL-6 and IL-1 beta directly into the brains of conscious rats to examine the effect of these cytokines on fever, and two behaviors affected by sickness, voluntary wheel-running and food intake. Male Sprague-Dawley rats selected for their predisposition to spontaneously run on running wheels were used in the experiment. Each rat was anaesthetized and had a temperature-sensitive radiotransmitter implanted intra-abdominally, and a 23-gauge stainless steel guide cannula inserted stereotaxically over the lateral cerebral ventricle. Rats were randomly assigned to receive intracerebroventricular injections of three doses of either IL-1 beta or IL-6 (100 ng, 1 ng or 0.1 ng IL-1 beta and 200 ng, 20 ng or 2 ng IL-6), or one of three different combinations of IL-1 beta and IL-6. Rats receiving either IL-1 beta or IL-6 showed a dose-dependent increase in body temperature and decrease in wheel-running (ANOVA, p<0.0001). Only rats receiving the highest dose of IL-1 beta significantly decreased food intake and body mass compared to rats receiving vehicle (ANOVA, p<0.001). Doses of IL-1 beta and IL-6 which, when injected on their own were non-pyrogenic and did not affect food intake and body mass, induced fever and anorexia when they were co-injected centrally. These results show that species-homologous rat IL-6 and IL-1 beta can act directly within the brain to decrease voluntary activity and suggest they also can act synergistically to induce anorexia and fever.

Activity, blood temperature and brain temperature of free-ranging springbok

Abstract We used miniature data loggers to record temperature and activity in free-ranging springbok (Antidorcas marsupialis) naturally exposed to severe nocturnal cold and moderate diurnal heat. The animals were active throughout the day and night, with short rests; the intensity of activity increased during daylight. Arterial blood temperature, averaged over many days, exhibited a circadian rhythm with amplitude <1 °C, but with a wide range which resulted from sporadic rapid deviations of body temperature. Peak blood temperature occurred after sunset. Environmental thermal loads had no detectable effect on blood temperature, even though globe temperature varied by >10 °C from day to day and >20 °C within a day. Brain temperature increased approximately linearly with blood temperature but with a slope <1, so that selective brain cooling tended to be activated at high body temperature, but without a precise threshold for the onset of brain cooling. Low activity attenuated selective brain cooling and high activity abolished it, even at high brain temperature. Our results support the concept that selective brain cooling serves to modulate thermoregulation rather than to protect the brain against heat injury.

Differences in the relative involvement of peripherally released interleukin (IL)-6, brain IL-1β and prostanoids in mediating lipopolysaccharide-induced fever and sickness behavior.

Although peripherally released interleukin (IL)-6 is critical for fever, its role in sickness behaviors, in particular anorexia and lethargy, induced by lipopolysaccharide (LPS) administration appears to be less important. Using quantifiable measures of fever, anorexia and lethargy, that is, body temperature, food intake and voluntary wheel-running, we investigated whether the less-than-essential role for IL-6 in mediating sickness behaviors compared to fever implies important roles for other inflammatory mediators, particularly IL-1β and prostanoids, in these responses. Male Sprague-Dawley rats were randomly assigned to receive one of the following three injections before receiving a subcutaneous (SC) injection of LPS (250 μg/kg) or saline: (1) intraperitoneal injection of pre-immune serum or antiserum to IL-6 (IL-6AS), to reduce the biological activity of peripherally released IL-6; (2) intracerebroventricular injection of vehicle or a caspase-1 inhibitor, to inhibit the production of mature IL-1β; or (3) intraperitoneal injection of vehicle or one of the two doses (1 or 10 mg/kg) of diclofenac, a nonselective cyclooxygenase inhibitor shown to block the formation of prostanoids. LPS administration induced fever, anorexia and lethargy with an accompanying increase in IL-6 and IL-1β concentrations in the circulation and IL-1β in the brain. Rats pre-treated with: (1) IL-6AS had reduced plasma levels of bioactive IL-6, no fever and attenuated sickness behaviors; (2) the caspase-1 inhibitor had reduced concentrations of IL-1β in the pre-frontal cortex, hypothalamus and hippocampus, and attenuated fever and sickness behaviors; (3) diclofenac had a dose-dependent attenuation in fever and sickness behaviors. Doses of diclofenac which completely abolished fever however had lesser effects on anorexia and lethargy. Our results confirm a difference in the sensitivity of sickness responses to IL-6 antagonism and identify that it may be related to different levels of sensitivity or responsiveness in brain regions and/or mechanisms, to prostanoids, IL-1β, or IL-6 itself.

Circulating cytokine concentrations and cytokine production by monocytes from newborn babies and adults

As a possible factor responsible for reduced fever responses in the newborn, we measured plasma cytokine concentrations and cytokine production by neonatal monocytes after lipopolysaccharide or IL (interleukin)-1α stimulation in vitro and compared these data with those obtained from adult plasma and monocytes. Whole blood was collected from afebrile adults (n=12) and the umbilical cord of normal term infants (n=12). Plasma and peripheral blood monocytes were prepared by conventional techniques. Significantly lower concentrations of IL-1α, IL-1β (P<0.05,t-test) and IL-6 (P< 0.01,t-test) were found in the plasma of newborn babies compared with that of adults. There was no significant difference in plasma tumour necrosis factor (TNF) concentrations between the adults and newborn babies. Monocytes from newborn babies had the capacity to produce IL-1α and IL-1β as readily as adult cells after stimulation with lipopolysaccharide or IL-1α, and produced significantly lower concentrations of TNF and IL-6 than those produced by stimulated adult monocytes (P< 0.01, ANOVA). Our results suggest that the reduced production of IL-6 by monocytes of the newborn during infection could be partly responsible for attenuated fever responses observed in the neonate.

Pyrogen and prostaglandin fever in the rabbit-I: Effects of salicylate and the role of cyclic AMP.

Abstract Intrahypothalamic injections of dibutyryl cyclic AMP into the rabbit produce a dose-dependent rise in rectal temperature with a short latency of onset. The fevers produced by intravenous administration of a bacterial pyrogen and intrahypothalamic injections of prostaglandin E 1 were both significantly potentiated by the simultaneous administration of theophylline, a nucleotide phosphodiesterase inhibitor. This suggests the possibility that both pyrogen and prostaglandin E 1 produce fevers via cyclic AMP. Sodium salicylate infused intravenously produced a marked attenuation of the pyrogen fever but was without effect on the prostaglandin E 1 fever. Since salicylate inhibits prostaglandin biosynthesis, this result is consistent with the idea that pyrogen fever is mediated via a prostaglandlin.

Effects of prostaglandin antagonism on sodium arachidonate fever in rabbits.

1. Sodium arachidonate, the prostaglandin precursor substance, when injected intraventricularly into rabbits, results in dose‐dependent hyperthermia, which is rapid in onset and of several hours duration. 2. Arachidonate fever was inhibited by intraventricular injection of indomethacin, but not by the simultaneous intraventricular injection of either of the two prostaglandin antagonists SC 19220 or HR 546. 3. Both antagonists effectively inhibited the fever induced by the intraventricular injection of an equipotent dose of PGE1. 4. Our results show that a derivative of arachidonic acid other than prostaglandin is pyrogenic.

Pyrogen and prostaglandin fever in the rabbit—II: Effects of noradrenaline depletion and adrenergic receptor blockade

Abstract Experiments were carried out to investigate a possible role for noradrenaline in the febrile response induced by intravenous injection of a purified bacterial pyrogen (0.02 μg) and by intrahypothalamic injection of prostaglandin E 1 (PGE 1 , 500 ng). Pretreatment of the hypothalamus of rabbits with 6-hydroxydopamine, which causes degeneration of noradrenergic nerve terminals, significantly reduced the febrile response to intravenous pyrogen injection. 6-Hydroxydopamine also caused a decrease in the febrile response to PGE 1 injected into the hypothalamus. In another series of experiments an intrahypothalamic injection of 50 μg of the alpha-adrenergic antagonist, phenoxybenzamine, significantly reduced pyrogen fever. Also, addition of 50 μg phenoxybenzamine to the PGE 1 injection, resulted in a significant reduction of the febrile response. Propranolol, a beta-adrenergic blocking agent, when injected in the same dose and in the same manner as phenoxybenzamine, had no significant effect on either the pyrogen- or the PGE 1 -induced fevers. It has previously been suggested that pyrogen acts by release of PGE 1 in the hypothalamus and results from our laboratory support this idea. In addition, our results suggest alpha-receptor activation in the genesis of both pyrogen- and PGE 1 -fever.

Interleukin-1 receptor antagonist in newborn babies and pregnant women

We have investigated the possible role of the interleukin-1 receptor antagonist (IL-1ra) in the attenuated fever response in the newborn. Umbilical cord blood was collected from normal full-term infants (n=12), and venous blood was obtained from afebrile, non-pregnant adults, of both genders (n=12) and women in late pregnancy (n=12). Plasma IL-1ra, and IL-1ra produced in vitro by peripheral blood monocytes stimulated with IL-1α or LPS, were assayed by ELISA. Significantly higher concentrations of IL-1ra (P<0.01, t test) were found in umbilical cord plasma than in plasma of non-pregnant adults. Furthermore concentrations of IL-1ra in the plasma of women in late pregnancy were significantly higher than in the plasma of neonates and non-pregnant adults (P<0.01, Mann-Whitney rank-sum test). Neonatal monocytes failed to produce significant amounts of IL-1ra upon stimulation in vitro. The monocytes of pregnant women produced much higher concentrations of IL-1ra than the monocytes of non-pregnant adults (P<0.01 Mann-Whitney rank-sum test). We speculate that IL-1ra may attenuate the febrile response to Gram-negative pyrogens in women in late pregnancy, and by crossing the placenta, also in the newborn.