Differential acute-phase responses in febrile and cold- and heat-exposed rabbits

Separate mechanisms inside and outside the blood-brain barrier inducing metabolic changes in febrile rabbits

1. We investigated the acute phase response induced by either intravenous (I.V.) or intracerebroventricular injections of bacterial endotoxin or endogenous pyrogen. These caused either monophasic or biphasic fever, and the response includes changes in plasma concentration of iron, zinc, copper, fibrinogen and in circulating leucocyte count.2. The I.V. injection of a small dose of endotoxin or endogenous pyrogen produced a monophasic fever, while a large dose produced a biphasic fever. The ventricular injection of endogenous pyrogen produced a fever similar to the second phase of the biphasic fever.3. The I.V. injection of a small dose of endotoxin or endogenous pyrogen produced a low plasma zinc 8 h after injection, while the ventricular injection of endogenous pyrogen produced a low plasma zinc 24 h after injection. The I.V. injection of a large dose of endotoxin or endogenous pyrogen induced a low plasma zinc 8 and 24 h after injection, suggesting that the hypozincaemia induced by the large dose was mediated by both peripheral and central action of endogenous pyrogen with different time courses.4. The I.V. injection of the small dose did not affect the level of the plasma copper concentration but the I.V. injection of the large dose and the ventricular injection increased it 24 h after injection. It is considered that the plasma copper concentration is mainly controlled by the central action of endogenous pyrogen.5. The changes in the plasma iron and fibrinogen concentration and the circulating white blood cell count induced by the different doses and by the different routes showed very similar patterns, indicating that these are simultaneously controlled by both peripheral and central actions of endogenous pyrogen.6. The present results show that there are two separate mechanisms involved in the acute phase response, one inside and one outside the blood-brain barrier. From the consideration that endogenous pyrogen released from the phagocytic leucocytes induces fever and acute phase response by its action on both the peripheral target organs and the central nervous system, it is suggested that endogenous pyrogen acts both centrally and peripherally, in the same manner as other hormonal agents such as corticosteroids.

Infection-induced anorexia: active host defence strategy

In association with fever production, decreased food consumption is the most common sign of infection. This effect is often regarded as an undesirable manifestation of sickness. However, evidence suggests that just as many behaviours have now been shown to modify immunocompetence, infection-induced anorexia is a behaviour systematically organised for pathogen elimination. That is, anorexia is an active defence mechanism that is beneficial for host defence. This review details the mechanism of infection-induced anorexia, placing it within the framework of the intricately organised acute phase response--the host response to infection. Furthermore, the evolutionary, behavioural, metabolic and immunological consequences of infection-induced anorexia are outlined, each providing evidence for the beneficial nature of this response. The evidence suggests that food restriction is one of the important behavioural strategies that organisms have evolved for the fight against pathogenic invasion. Nevertheless, such benefits require fine homeostatic control, as chronic undernutrition has deleterious consequences for host defence.

Behavioral conditioning of endotoxin-induced plasma iron alterations

The cascade of physiologic mechanisms in response to infection, the acute-phase response, is recognized as playing a major role in host defence. One such response is the hypoferremia that is consistently reported to occur during bacterial infection. This study aimed to determine whether the alterations in plasma iron were conditionable using the conditioned taste aversion (CTA) paradigm. The regime involved the pairing of a novel-tasting saccharin solution with bacterial endotoxin. Seven days after the initial pairing of these stimuli (the test day), the saccharin solution was represented. Animals exposed to this condition displayed a significant reduction in the level of plasma iron. Animals treated with an intraperitoneal dose of 400 micrograms/Kg lipopolysaccharide (LPS) displayed lower conditioned iron levels than rats infused with 100 micrograms/Kg LPS; however, this difference was not significant. These results showed that in addition to other acute-phase responses (fever and anorexia), plasma iron alterations are able to be manipulated through behavioral manipulations.

Effect of ambient temperature and E. coli endotoxin upon the plasma iron level in wild house mice in winter season

The effects of ambient temperatures of 10 degrees C and 30 degrees C and of E. coli endotoxin on brain temperature and plasma iron level were investigated in unrestrained wild house mice, Mus musculus. In control animals (i.p. saline-injected) exposed to cold environment the brain temperature decreased and plasma iron levels were lower than those observed under thermoneutral conditions (30 degrees C). Animals injected i.p. with endotoxin (0.5 micrograms.kg-1) and placed at 30 degrees C showed a drop in plasma iron level during the fever episode. The results provide strong evidence for a relationship between brain temperature and plasma iron level in control mice under thermoneutral conditions, and show that during cold exposure or after injection of endotoxin, there is no linear correlation between brain temperature and plasma iron. Moreover, it was found that cold stress influences plasma iron level and that this influence is not mediated by changes in brain temperature.

Brain regions involved in the development of acute phase responses accompanying fever in rabbits

1. The effects of microinjection of rabbit endogenous pyrogen and human recombinant interleukin-1 alpha on rectal temperature and acute phase responses were extensively examined in forty different brain regions of rabbits. The acute phase responses that were investigated were the changes in plasma levels of iron, zinc and copper concentration and the changes in circulating leucocyte count. 2. The rostral hypothalamic regions, such as nucleus broca ventralis, preoptic area and anterior hypothalamic region, responded to the microinjection of endogenous pyrogen or interleukin-1 by producing both fever and acute phase responses. 3. The microinjection of endogenous pyrogen or interleukin-1 into the rostral hypothalamic regions significantly decreased the plasma levels of iron and zinc concentration 8 and 24 h after injection. The circulating leucocyte count increased 8 h after injection. However, neither the injections of endogenous pyrogen nor interleukin-1 affected the number of red blood cells. 4. The present results show that the rostral hypothalamic regions respond directly to endogenous pyrogen or interleukin-1 with the consequent development of fever and acute phase responses.

Somnogenic activities of synthetic lipid A

Bacterial infections and various immune response modifiers, including endotoxin and its lipid A moiety, alter sleep duration. The purpose of this study is to amplify our understanding of lipid A structure-somnogenic-pyrogenic activity relationships. Four synthetic disaccharide analogs of lipid A (LA-15-PP, LA-15-PH, LA-16-PH, and LA-18-PP) and a biosynthetic monosaccharide analog of lipid A (lipid X) were tested in rabbits for their effects on slow-wave sleep, rapid-eye-movement sleep, electroencephalographic slow-wave (0.5- to 4.0-Hz) amplitudes, and brain-colonic temperatures. Substances were injected intravenously and intracerebroventricularly. Compound LA-15-PP was the most potent; it significantly increased slow-wave sleep, delta electroencephalographic amplitudes, and brain-colonic temperatures while reducing rapid-eye-movement sleep. Compound LA-15-PH (monophosphoryl analog of LA-15-PP) induced effects similar to those of LA-15-PP, although the responses were weaker. Compound LA-18-PP induced increases in slow-wave sleep and delta amplitudes only after high doses, whereas compound LA-16-PH was devoid of these activities at the doses tested. Intracerebroventricular, but not intravenous, injections of lipid X induced small but significant increases in both slow-wave sleep and rapid-eye-movement sleep without affecting delta amplitudes or brain-colonic temperatures. These data suggest that the somnogenic actions of these lipid A analogs depend on the acylation or phosphorylation pattern and backbone structures of the molecules. Their soporific activities parallel their relative behaviors in other biological assays.

Leukocytosis induced by microinjection of endogenous pyrogen or interleukin-1 into the preoptic and anterior hypothalamus

We investigated the effect of microinjection of endogenous pyrogen or interleukin-1 (EP/IL-1) into 40 selected brain regions on the circulating leukocyte count in rabbits. The results show that injections into the rostral region of the hypothalamus, especially the preoptic and anterior hypothalamus, induce leukocytosis.

Thermal stimulation of the hypothalamus does not evoke the acute-phase reaction

Interleukin-1 (IL1) injected into the preoptic-anterior hypothalamus (POAH) induces, besides fever, the hepatic synthesis of acute-phase glycoproteins. Since the febrigenic action of IL1 may involve thermosensitive neurons in the POAH, this study examined whether such neurons also might mediate the acute-phase response (APR). The POAH of six adult NZW rabbits was cooled (Tpo = 34.4 +/- 0.4 degrees C [mean +/- SD]) or heated (40.6 +/- 0.2 degrees C) continuously for 2.5 hr (so as to mimic the mean febrile course following a bolus microinjection of IL1 into the POAH). The ambient temperature (Ta) was 23.5 +/- 1.0 degrees C. Expectedly, core temperature fell and skin temperature rose on POAH heating, and the opposite occurred on POAH cooling. However, no statistically significant changes in the plasma levels of Fe, Zn, Cu, and N-acetylneuraminic acid, as indices of the APR, were induced by these treatments. These results indicate, therefore, that the central actions of IL1 in inducing fever and the APR are separate, and that the APR is not mediated through stimulation of thermosensitive units in the POAH.

Thermal and acute-phase protein responses of guinea pigs to intrapreoptic injections of leukotrienes

Although it seems probable that intrahypothalamic prostaglandin (PG) E2, a cyclooxygenase metabolite of arachidonic acid, modulates interleukin-1 (IL1)-induced fever, the evidence that it plays such a role is still only circumstantial; PGE2 does not, however, centrally mediate the fever-associated, acute-phase glycoprotein response. In this study, we investigated whether lipoxygenase products of arachidonic acid, viz. leukotriene (LT) B4, C4, D4 OR E4, injected intrapreoptically (2 ng/microliter, 1 microliter bilaterally) induces, like IL1, febrile and acute-phase glycoprotein responses in guinea pigs; controls received pyrogen-free saline, IL1 or PGE2. Measurements were: core temperature (Tco) and, as indices of acute-phase glycoproteins, plasma levels of copper (Cu) and protein-bound N-acetylneuraminic acid (NANA). Unlike IL1 or PGE2, no LT caused a febrile rise in Tco. Similar to PGE2 but unlike IL1, no LT produced increases in the plasma levels of Cu and NANA. These results indicate that intrapreoptic LTs probably are not involved in initiating the febrile or acute-phase glycoprotein responses characteristics of IL1.

Hypothalamic opioids and the acute-phase glycoprotein response in guinea pigs

Endogenous opioids (EO) probably do not modulate endotoxin (LPS)- or interleukin 1 (IL1)-induced fever because naloxone does not prevent its development. Yet, increases in CSF and hypothalamic levels of beta-endorphin have been reported during LPS-and IL1-induced fevers. Since IL1 also reduces the specific binding of opioids to their receptors in guinea pig brain, the opioids could be involved in modulating nonfebrile effects of IL1. To determine whether EO might have a role in the IL1-induced acute-phase glycoprotein response of guinea pigs, (1) naloxone (5 and 10 mg/kg, SC) was injected prior to LPS (S. enteritidis 2 micrograms/kg, IV; N = 5), and (2) morphine (MOR, 10 micrograms/microliter), [D-ala2]-met-enkephalinamide (DAME, 5 micrograms/microliter), or dynorphin A (DYN, 5 micrograms/microliter) was injected into the preoptic area (1 microliter, bilaterally; N = 8/treatment) or into the 3rd ventricle (N = 4/treatment); pyrogen-free saline was the control injection. Measurements were: core temperature (Tco) and, as indices of acute-phase glycoproteins, plasma levels of copper (Cu) and N-acetylneuraminic acid (NANA). Naloxone did not prevent the fever or the increases in plasma Cu and NANA levels evoked by LPS. The intracerebral administration of opioid agonists by either route induced variable rises in Tco, each with a different pattern, but no increases in plasma Cu and NANA levels. Thus, EO do not participate in the central modulation of acute-phase glycoprotein synthesis, but may have a role in influencing other nonthermal IL1 effects in the CNS.