Central injection of IL-10 antagonizes the behavioural effects of lipopolysaccharide in rats
Introduction
When an organism becomes sick during the course of an infection, several changes occur, which are mediated by the central nervous system (CNS). These changes include regulated increases in body temperature, sleep, activation of the hypothalamic-pituitary-adrenocortical (HPA) axis, decreases in locomotor activity, feeding, drinking and social interactions, and alterations in brain neurotransmitters (Dantzer et al., 1996). They are due to the release of proinflammatory cytokines such as interleukin-1 (IL-1), IL-6 and tumor necrosis factor-α (TNFα) by activated monocytes and macrophages. The same effects can be obtained by systemic administration of the cytokine-inducer lipopolysaccharide (LPS) and they are mediated by induction of the expression of cytokines in the brain (Kent et al., 1992, Gatti and Bartfai, 1993, Breder et al., 1994, Layé et al., 1994, Quan et al., 1998).
LPS induces the production of not only pro-inflammatory but also anti-inflammatory cytokines such as IL-4, IL-10, IL-13 and transforming growth factor-β (TGF-β). Anti-inflammatory cytokines have the ability to suppress the synthesis of IL-1, TNF, and other cytokines in peripheral immune and non immune cells (Dinarello, 1997a, Dinarello, 1997b, Marie and Cavaillon, 1997). IL-10 is a 35–40 kDa protein produced by CD4+, Tho, Th1, B lymphocytes, mast cells, eosinophils, monocytes and macrophages. IL-10 actions vary according to the cell type, the nature of the stimulus and the cellular environment (Moore et al., 1993, Lalani et al., 1997). In vitro, IL-10 inhibits monocyte production of several proinflammatory mediators, including IL-1, IL-6, IL-8, granulocyte colony-stimulating factor and TNFα, and it upregulates the expression of the IL-1 receptor antagonist (IL-1ra) (Howard and O’Garra, 1992, Kasai et al., 1997). In vivo, systemic administration of pharmacological doses of IL-10 inhibits TNF release and protect mice from LPS-induced lethality (Gérard et al., 1993, Marchant et al., 1994). The ability of IL-10 to suppress the production of inflammatory cytokines while at the same time upregulating the anti-inflammatory agent IL-1ra suggests that it may be a potent anti-inflammatory molecule in a variety of diseases including septic shock. Several experiments have shown that IL-10 is active at the CNS level. Indeed, intracerebroventricular (ICV) injection of IL-10 alters cortical EEGs and decreases slow wave sleep, the opposite effect of IL-1 (Opp et al., 1995). IL-10 is also able to downregulate the expression of cytokines in the brain. ICV injection of murine recombinant IL-10 in mice completely inhibits brain IL-1β and TNFα production in response to ICV LPS (Di Santo et al., 1995). In addition, intravenous administration of recombinant human IL-10 inhibits brain TNFα production in response to ICV LPS in mice (Di Santo et al., 1997). Since the behavioural symptoms that develop in rodents injected with systemic or central injections of LPS are mediated by the synthesis and release of proinflammatory cytokines in the brain (see Dantzer et al., 1996 for a review), it might be predicted that ICV IL-10 should be able to abrogate these behavioural effects. The present experiment was specifically designed to investigate this possibility in the rat using recombinant rat IL-10. We show that ICV IL-10 blocks the behavioural effects of IP or ICV LPS.
Section snippets
Animals
Male Wistar rats (125–150 g, Charles River, St-Aubin-lès-Elbeuf, France), were housed in groups of five in polypropylene cages under a 12:12 h light:dark cycle (lights on at 21:00 h). Room temperature was 23±1°C. Food and water were available ad lib. Juveniles of the same strain served as stimulus animals and were housed in groups of ten.
Surgery
After a 2-week acclimatization period, rats were anaesthetized IP with a mixture of ketamine and xylazine (61 mg and 9 mg/kg, respectively), and were equipped
Experiment 1: effects of ICV IL-10 on IP LPS
As expected, IP LPS decreased the duration of social exploration for the first 6 h of observation with a full recovery at 24 h, and induced immobility at 4 and 6 h. It also induced body weight loss (Fig. 1, Fig. 2).
A three-way ANOVA on the duration of social investigation revealed a significant effect of LPS (F[1,30]=21.8, p<.001), and time (F[4,30]=23.5, p<.001) factors, and significant LPS×time (F[4,120]=18.8, p<.001), and IL-10×LPS×time (F[12,120]=3.0, p<.001) interactions. ICV IL-10 had no
Discussion
The present results demonstrate for the first time that ICV administration of IL-10 is able to attenuate the behaviourally depressing effects of LPS whether this cytokine inducer is administered at the periphery or centrally. Since the effects of LPS are mediated by the local production of proinflammatory cytokines (Moldawer et al., 1987, Zuckerman et al., 1989) which, in the case of systemically injected LPS, induce the expression of proinflammatory cytokines in the brain (Kent et al., 1992,
Acknowledgements
The authors are grateful to Viviane Tridon and Jean-Pierre Villars for technical assistance. This research was supported by INSERM, INRA, the European Community (Cybrainet, Biomed 2, CT97-2492) and by a Grant from the National Institutes of Health MH-51569 (KWK).
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