Effects of pyrogen on the medullary temperature-responsive neurone of rabbits

Interleukin-1 beta induces histamine release in the rat hypothalamus in vivo

We have previously demonstrated the increase of histidine decarboxylase activity and histamine content in the murine hypothalamus after intracerebroventricular injection of lipopolysaccharide possibly due to inducible interleukin-1 beta (IL-1 beta). Therefore, we investigated the effects of IL-1 beta on brain histamine dynamics by directly injecting it into the tuberomammillary nucleus of the rat hypothalamus (TM) using an in vivo microdialysis method. Injection of artificial cerebrospinal fluid or recombinant murine IL-1 beta at 0.1 ng into the TM did not evoke a significant change in core temperature, however, a significant monophasic febrile response was observed following injection of IL-beta at more than 1 ng per animal. Histamine release in the anterior hypothalamic area in vivo was significantly augmented from 140 min to 360 min following injection of IL-1 beta at 10 ng dose. These results suggest the possibility that interrelationship between histamine and IL-1 beta may modulate the acute phase reaction in the central nervous system.

Neural mechanisms in the pyrogenic and acute-phase responses to interleukin-1

It has become increasingly apparent over the past several decades that the hypothalamus, among other brain regions, plays an important part in the modulation of the immune system (reviewed in Korneva et al., 1985; Roszman & Brooks, 1985; Jankovic & Spector, 1986; Cotman et al., 1987). Since the hypothalamus also mediates the fever and various other acute-phase responses characteristic of the early stages of infection (reviewed in Hellon & Townsend, 1983; Blatteis, 1984, 1985; Cooper, 1987), it is possible that the localization within a common brain region of the controllers of several, different host defense reactions is not a happenstance, but represents a highly organized neuronal network serving to coordinate them. Indeed, pyrogenic, inflammatory, and immune responses do interact in the defense of the host against infection (reviewed in Dinarello, 1984). It is not yet known how immune responses are integrated centrally, but some data are available on the neural mechanisms controlling fever and certain components of the acute-phase reaction. The purpose of this paper is to review these briefly in the hope that a background can be provided against which features that may be common to neuroimmunomodulation and to the control of acute-phase reactions might be revealed.

Thermosensitivity of neurons in the paraventricular nucleus of the rat slice preparation

The thermosensitivity of 65 spontaneously active neurons in the paraventricular nucleus (PVN) was investigated by extracellular recording in the rat hypothalamic slice preparation. The firing rate of these cells was comparatively low, ranging from 0.03 to 10.0 (mean 2.46) impulses/s at 37 degrees C, and only a minority showed a phasic firing pattern. Of 65 neurons tested, 23 (35%) increased their firing rate when the slice was warmed (warm-sensitive neurons) and 9 (14%) showed the opposite response (cold-sensitive neurons). Thermosensitivity was also tested in solutions with reduced [Ca2+] and high [Mg2+]. Eight out of 10 warm-sensitive neurons and 5 of 7 cold-sensitive neurons retained thermosensitivity after synaptic blockade. Out of 6 phasic firing neurons tested, one showed warm-sensitivity and another one showed cold-sensitivity. The thermosensitive neurons were diffusely distributed throughout the PVN and were not located in particular areas of the nucleus. Thus a group of cells in the PVN, including probably both magno- and parvocellular neurons, showed an inherent thermosensitivity, which suggests an important role for the PVN in thermoregulation.

Examination of the subdiencephalic rat brain for sites mediating PGE1-induced pyrexia

The subdiencephalic rat brain was mapped for sites capable of mediating prostaglandin-induced pyrexia. In conscious rats, PGE1, 200 ng in a volume of 1 microliter, was injected unilaterally into 412 sites between the midmesencephalon and the caudal medulla. Injections into only 12 sites caused a reproducible, short-latency core temperature increase of at least 0.5 degrees C. None of these was located in the paramedian brainstem, which was considered a likely site of PGE1 action because of the presence there of thermosensitive and pyrogen-sensitive neurons. Rather, the reactive loci were found in the hippocampus (5 sites) and in the vicinity of the cochlear nuclei (7 sites). Injections into only 2 sites in the latter region failed to produce pyrexia. In the hippocampus, however, injections at 31 sites in the same frontal planes as the reactive loci produced no effect. The possibility that the active hippocampal sites were associated with a distribution of injectate to PGE1-sensitive neurons located within hippocampal cleavage planes rather than in a circumscribed region is discussed.

Autonomic thermoregulation after separation of the preoptic area from the hypothalamus in rats

To clarify the role of the hypothalamic preoptic area in autonomic thermoregulation, the preoptic area (POA) of rats was disconnected from the rest of the brain-stem by bilateral microknife cuts which spared or included the medial forebrain bundle. The animals' metabolic responses to exogenous norepinephrine (0.5 mg/kg, im) were then measured at ambient temperatures of 25 degrees and 15 degrees C. Oxygen uptake and colonic and tail skin temperatures were also measured at ambient temperatures of 34 degrees, 25 degrees, and 15 degrees C. Finally, the febrile response to a challenge with live Salmonella enteritidis was studied. Except for a slightly higher oxygen uptake at all ambient temperatures in the rats in which the medial forebrain bundle was cut, no differences were found in any of the variables studied between the POA-disconnected and the sham-operated animals. We conclude, therefore, that the POA is not essential for the integration of autonomic thermoregulatory responses in the rat.

Molecular mechanisms in endotoxin fever

Two important concepts are presented in this review. First, endotoxin fever, like all fevers, is mediated by a host product, leukocytic pyrogen (LP). The mechanism by which LP production is initiated by endotoxin is discussed and evidence is provided which clearly distinguishes the biological and physical differences between LP and endotoxins. The second concept is that many of the molecular and neurochemical mechanisms by which LP causes fever by its action on the hypothalamic thermoregulatory center are also observed when endotoxins are introduced into the central nervous system. Thus, there may be experimental and clinical situations in which endotoxins can directly affect the hypothalamus and initiate fever. Although this bi-modal effect of endotoxin on the production of fever can occur, the importance of LP in mediating endotoxin and other fevers cannot be overstated.

Molecular basis of fever in humans

This review presents several areas of research on the pathogenesis of fever in humans and updates new information concerning the role of fever in host defense mechanisms. Fever is mediated by a polypeptide of phagocytic cell origin called leukocytic pyrogen. Several agents and disease processes are associated with the synthesis and release of leukocytic pyrogen. Although the original studies on leukocytic pyrogen suggested that the neutrophil was the primary source, recent experiments indicate the mononuclear phagocyte to be the major producer of leukocytic pyrogen. The mechanism by which human monocytes are stimulated to produce leukocytic pyrogen is discussed, including the effects of corticosteroids, estrogens and antipyretics on the synthesis of leukocytic pyrogen in vitro. The ability of leukocytic pyrogen to alter the hypothalamic thermoregulatory center by increasing arachidonic acid metabolite levels is the most likely mechanism by which leukocytic pyrogen initiates fever. Antipyretics prevent the synthesis of certain cyclooxygenase metabolites, which accounts for their ability to reduce fever. Studies on the chemical and physical properties of human leukocytic pyrogen are reviewed and form the basis for current experiments on the similarities between leukocytic pyrogen and lymphocyte activating factor. These studies suggest that leukocytic pyrogen, in addition to producing fever, also stimulates non-hypothalamic cells involved in aspects of the acute-phase response. In this regard, leukocytic pyrogen may be an important mechanism for host defenses. Hyperthermia may also be beneficial to the host but is distinct from fever; the role of leukocytic pyrogen as well as hyperthermia as a defense mechanism is discussed.