Prior exposure to glucocorticoids sensitizes the neuroinflammatory and peripheral inflammatory responses to E. coli lipopolysaccharide☆
Introduction
A considerable literature has shown that the neuroendocrine, neurochemical, and neuroinflammatory sequelae of stress and pro-inflammatory immune activation are not only similar, but also cross-sensitize one another (Anisman, 2009, Tilders and Schmidt, 1999). Acute and chronic stress has been found to sensitize or prime the neuroinflammatory response to both peripheral and central immunologic challenges (de Pablos et al., 2006, Espinosa-Oliva et al., 2009, Frank et al., 2007, Johnson et al., 2002, Johnson et al., 2003, Johnson et al., 2004, Munhoz et al., 2006). For example, both exposure to a single session of inescapable tailshock (Johnson et al., 2002) and to 14 daily sessions of unpredictable chronic stress (Munhoz et al., 2006) potentiate the increase in hippocampus and frontal cortex expression of pro-inflammatory mediators (i.e. interleukin-1β; IL-1β, inducible nitric oxide synthase, tumor necrosis factor-α) produced by a peripheral injection of lipopolysaccharide (LPS) given 24 h after the stressor regimen. Several studies have also shown that both acute and chronic stress activates microglia including up-regulated MHCII (de Pablos et al., 2006, Frank et al., 2007) and F4/80 antigen (Nair and Bonneau, 2006), suggesting that stress shifts the neuroimmune microenvironment towards a pro-inflammatory immunophenotype. Interestingly, stress-induced microglia activation as well as sensitization of neuroinflammatory processes were blocked by a glucocorticoid (GC) receptor antagonist (de Pablos et al., 2006, Espinosa-Oliva et al., 2009, Munhoz et al., 2006, Nair and Bonneau, 2006), suggesting that GCs may mediate stressed-induced sensitization of neuroinflammatory processes. Although these data suggest that GCs are somehow involved, whether GCs are sufficient to prime neuroinflammatory responses has not been systematically investigated.
Though GCs are universally considered anti-inflammatory (Boumpas et al., 1993), there is emerging evidence that under some conditions GCs sensitize central pro-inflammatory responses (see (Sorrells and Sapolsky, 2007) for review). Although the key determinants are not clear, it should be noted that the immune challenge (LPS) was administered after the onset or termination of the stressor regimen in the studies in which acute and chronic stress induced an exaggerated neuroinflammatory response to an immune challenge. Interestingly, if LPS is administered immediately before stress exposure, stress exhibits an anti-inflammatory effect, including inhibition of LPS-induced increases in brain IL-1β (Goujon et al., 1995). Clearly, the effects of stress on inflammatory responses to challenge would appear to depend, in part, on the timing between the stressor and the challenge. If GCs mediate these effects of stress, then timing between GC elevation and challenge should be critical as well. Since there is abundant evidence that elevated GCs are anti-inflammatory while they are present, GCs might be expected to be pro-inflammatory only after levels have returned to baseline. Thus, here we tested whether acute administration of exogenous GCs would be sufficient to reproduce the stress-induced sensitization of neuroinflammatory responses under a number of different timing relationships between GC administration and challenge (LPS). Since it has been suggested that the pro-inflammatory effects of GCs are limited to the brain (Munhoz et al., 2006), we assessed both brain and liver. The liver was chosen because it is a key peripheral organ mediating inflammation (Gao et al., 2008). We demonstrate here that GCs potentiate both the peripheral (liver) and central (hippocampus) pro-inflammatory response to a peripheral immune challenge (LPS) if GCs are administered prior (2 and 24 h) to challenge. Prior exposure (24 h) to GCs also potentiated the pro-inflammatory response of hippocampal microglia to LPS ex vivo. In contrast, when GCs are administered after (1 h) a peripheral immune challenge, GCs suppress the pro-inflammatory response to LPS in both liver and hippocampus.
Section snippets
Animals
Male Sprague–Dawley rats (60–90 day-old; Harlan Sprague–Dawley, Inc., Indianapolis, IN, USA) were pair-housed with food and water available ad libitum. The colony was maintained at 25 °C on a 12-h light/dark cycle (lights on at 07:00 h). All experimental procedures were conducted in accord with the University of Colorado Institutional Animal Care and Use Committee.
Reagents
Corticosterone (CORT; Sigma, St. Louis, MO) was dissolved in 100% propylene glycol. Lipopolysaccharide (LPS; Escherichia coli serotype
CORT does not activate TLR4 signaling
LPS has been found to contaminate a wide variety of commercial-grade preparations (Weinstein et al., 2008). LPS levels are typically not reported for commercially available GCs. Given the present goal of testing for the pro-inflammatory properties of GCs, we screened our CORT samples for LPS contamination to eliminate this potential confound. Utilizing TLR4 transfected HEK cells, we tested whether the CORT samples obtained from Sigma would activate TLR4 signaling. TLR4-HEK cells are highly
Discussion
GCs are generally regarded as universally anti-inflammatory, and indeed have a variety of actions that inhibit inflammation. For example, GC-bound GC receptors directly interfere with the transcriptional activity of the pro-inflammatory transcriptional factors nuclear factor kappa B and activating protein 1, thereby decreasing the expression of a host of inflammatory genes such as IL-1β (De Bosscher et al., 2003). However, the literature contains a fairly sizable number of instances in which
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Please see Brief Commentary by Shawn F. Sorrells and Robert M. Sapolsky in this issue.