LPS-Induced Sickness Behavior Is Regulated by TLR 4 but also by Other Mechanisms

Abstract

One characteristic of sickness behavior is demonstrated by a reduction in voluntary wheel-running activity during infection. Lipopolysaccharide (LPS), known as endotoxin, is a component of the gram-negative bacteria that induces sickness behavior and increases proinflammatory cytokines. LPS, which triggers the biosynthesis of diverse inflammatory mediators, is comprised of an O-poly saccharide repeating unit, a core-oligosaccharide, and lipid A. Although lipid A binds to toll-like receptor (TLR) 4 directly, LPS also binds to other binding molecules (e.g. LPS binding protein, soluble and membrane CD14, and TLR2). However, it is not clear whether these factors are also important in mediating LPS-induced sickness behavior. PURPOSE: The purpose of this study was to determine TLR4 was solely responsible for reduced running wheel behavior in mice. Our hypothesis was that LPS-induced sickness behavior is regulated both through TLR4 and by other means. MeTHOdS: Male C3H/HeJ mice (n=11), which are tlr4-gene mutated, were assessed for wheel-running activity before and after LPS (1mg/kg) injection. We also measured LPS-induced changes in wheel-running activity in male C3H/HeN mice (which possess functional TLR4), which were injected i.v. with LPS from Salmonella minnesota (1mg/kg, n=8), lipid A from Salmonella minnesota (1mg/kg, n=8) or saline (n=6). We also examined the effect of LPS and lipid A on in vitro TNF-□ production from peritoneal macrophages in C3H/HeN and C3H/HeJ mice. ReSUlTS: Wheel-running activity in C3H/HeJ mice was slightly, but significantly reduced after LPS injection (from 6,636+/−530 to 4,657+/−524 revolutions/day, 30% reduction, p<0.01). Wheel activity in LPS injected C3H/HeNmice was significantly lower than that in saline injected group (512+/−91 and 9,531+/−657 revolutions/ day, respectively, 95% reduction, p<0.01). Interestingly, the activity in lipid A injected mice was significantly higher than that in LPS injected mice (2,851+/−540 revolutions/day, 70% reduction, p<0.01 vs. LPS group), although Lipid A injected mice showed significant reduction of the activity (p<0.01 vs. saline group). Thus, lipid A is responsible for some but not all of the LPS-induced reduction in wheel-running behavior. In agreement with the running wheel data, macrophage TNF-□ also followed a similar pattern (e.g. highest in LPS treated, lower in lipid A treated cultures). CONClUSiON: Our results suggest that the transient reduction in physical activity after LPS injection is mediated via TLR4 directly (∼70%), but also by other binding molecules (∼30%).