Elsevier

Brain Research

Volume 1023, Issue 1, 8 October 2004, Pages 48-53
Brain Research

Research report
Bacterial endotoxin induces STAT3 activation in the mouse brain

https://doi.org/10.1016/j.brainres.2004.06.076Get rights and content

Abstract

In the present study, we investigated regulatory mechanisms of bacterial endotoxin-induced STAT3 activation in the brain. Intraperitoneal injection of lipopolysaccharide (LPS) dose-dependently (0.5–5000 μg/kg) induced STAT3 phosphorylation in the hypothalamus. LPS-induced STAT3 phosphorylation was peaked at 2–4 h and declined there after. Moreover, intracerebroventricular injection of LPS induced STAT3 phosphorylation in the cortex and the hippocampus, indicating that central as well as peripheral LPS can act in the brain to induce STAT3 activation. Glucocorticoids are known to play a physiological role in the feedback inhibition of immune/inflammatory responses in the endocrine system. Interestingly, we observed no effect of dexamethasone on LPS-induced STAT3 phosphorylation in the hypothalamus. These findings point to the important role of STAT3 in the neuroimmune interaction of inflammation in the brain.

Introduction

The central nervous system (CNS) has been regarded as an immunologically privileged site. However, recent evidence suggests that the CNS can operate certain immune functions [16], [17], [19], [20], [21], [35]. The CNS affects the peripheral immune system through endocrine, paracrine and neuronal mechanisms. On the other hand, the peripheral immune system communicates with the CNS. Peripheral inflammation can stimulate a variety of physiological responses mediated through the CNS, including fever [27], hypothalamic–pituitary–adrenal (HPA) axis activation [24] and sickness [26]. Glucocorticoids are hormones produced in the endocrine system in response to HPA axis activation and have the profound effects on the immune system of the CNS as well as peripheral [7]. Glucocorticoids are known to down-regulate the expression of pro-inflammatory cytokines such as IL-1 and IL-6 [2], [21], [33], [42]. The mechanisms by which the peripheral immune system transmit signals to the brain are unknown, but several mechanisms have been proposed [20], [29]. The mechanisms include: (1) direct entry of cytokine into the brain across the blood–brain barrier [4], (2) interaction of cytokine with circumventricular organs which lack the blood–brain barrier [25] and (3) activation of afferent neurons of the vagus nerve to transmit peripheral inflammatory signals to the brain [16], [45].

To better understand the regulatory mechanisms of cytokine-to-brain communication in infection and inflammation, we measured the brain signal transducer and activator of transcription 3 (STAT3) phosphorylatin as a marker for the activation mediated through several cytokines such as Interleukin (IL)-6, IL-10, leukemia inhibitory factor (LIF) and leptin. STAT3 is activated through tyrosin phosphorylation mediated by receptor-associated kinases Janus kinase (JAK) family proteins. In the present study, we investigated the site of action (peripherally vs. central) of lipopolysaccharide (LPS)-induced STAT3 phosphorylation in the brain. Moreover, we investigated the role of glucocorticoids on LPS-induced STAT3 phosphorylation in the brain.

Section snippets

Animals

C57BL/6J mice were obtained at 7 weeks old from Charles River (Japan). Mice were maintained in a room at 22–24 °C under a constant day–night rhythm and given food and water, ad libitum. All animal experiments were carried out in accordance with the NIH Guide for Care and Use of Laboratory Animals and approved by the animal care and use committee at Hokkaido University.

LPS injection and sample preparation

For peripheral injection, LPS (055:B5, Sigma) and dexamethasone–water soluble (Sigma) were dissolved in saline, and all

Dose response and time course study of peripheral LPS-induced STAT3 phosphorylation in the hypothalamus

We observed LPS (5 mg/kg, i.p.)-induced STAT3 phosphorylation from 2 h, which peaked at 4 h and declined thereafter (Fig. 1). We also observed STAT3 phosphorylation in the cortex and the hippocampus 4 h after peripheral LPS injection (data not shown). LPS-induced STAT3 phosphorylation was still significantly increased at 24 h (Fig. 1). Interestingly, we did not observe an increase in STAT3 phosphorylation at 30 min after LPS injection, suggesting the possibility that LPS-induced STAT3

Discussion

There is growing evidence that bi-directional regulation of the CNS and the peripheral immune system occurs in response to infection [16], [20], [35]. In the CNS, detection of pathogens by glial cells triggers these cells to synthesize and release proinflammatory cytokines such as IL-1β, IL-6 and TNF-α to generate an immune response to infection [6]. In the present study, we found a central as well as peripheral application of LPS-induced STAT3 phosphorylation in the brain. Glucocorticoids are

Acknowledgements

This research was supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan (Y.O., Y.N.), and Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists (T.H.).

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