Systemic immune activation leads to neuroinflammation and sickness behavior in mice

Abstract

Substantial evidence indicates an association between clinical depression and altered immune function. Systemic administration of bacterial lipopolysaccharide (LPS) is commonly used to study inflammation-associated behavioral changes in rodents. In these experiments, we tested the hypothesis that peripheral immune activation leads to neuroinflammation and depressive-like behavior in mice. We report that systemic administration of LPS induced astrocyte activation in transgenic GFAP-luc mice and increased immunoreactivity against the microglial marker ionized calcium-binding adapter molecule 1 in the dentate gyrus of wild-type mice. Furthermore, LPS treatment caused a strong but transient increase in cytokine levels in the serum and brain. In addition to studying LPS-induced neuroinflammation, we tested whether sickness could be separated from depressive-like behavior by evaluating LPS-treated mice in a panel of behavioral paradigms. Our behavioral data indicate that systemic LPS administration caused sickness and mild depressive-like behavior. However, due to the overlapping time course and mild effects on depression-related behavior per se, it was not possible to separate sickness from depressive-like behavior in the present rodent model.

Figure 1

Intraperitoneal administration of LPS caused a dose- and time-dependent increase in brain bioluminescent signal in GFAP-luc transgenic mice (a). A clear LPS-induced bioluminescent signal was visible in the brain, as seen on representative images taken from animals treated with different doses of LPS at 6 hours after injection (b). The color on the image represents the number of photons emitted from the animal per second, as indicated in the color scale on the right. Graphs are plotted as mean + SEM (n = 8 per group). Data were analyzed by rmANOVA followed by independent samples t-test. **P < 0.01 compared to 0 mg/kg LPS.

Figure 2

Peripheral LPS injection (0.63 mg/kg, i.p.) increased IBA1 immunoreactivity, a marker of microglial activation, in the hippocampal dentate gyrus at 24 h after administration. Representative images (10x) (a), quantified images of n = 10 per group (b). Graph is plotted as mean + SEM. Data were analyzed by ANOVA followed by independent samples t-test. ***P < 0.001 compared to vehicle.

Figure 3

Peripheral LPS administration transiently increased cytokine levels in serum and brain. Comparison of selection of cytokines and one chemokine (MCP-1) in serum (left) and brain (right) after i.p. LPS administration. Dashed lines indicate the detection limit of measured cytokine. Note that serum concentrations are expressed in pg/mL, while brain levels are shown in pg/mg protein. Graphs are plotted as mean + SEM (n = 12 per group). Data were analyzed by ANOVA followed by independent samples t-test. *P < 0.05, **P < 0.01, ***P < 0.001 compared to 0 mg/kg LPS.

Figure 4

Intraperitoneal injection of LPS caused sickness, but no clear depressive-like behavior is observed. Peripheral immune activation caused a dose- and time-dependent reduction in locomotor activity in the open field test (OFT). However, a single i.p. injection of LPS did not induce clear anxiety or depressive-like behavior in the stress-induced hyperthermia (SIH) test, tail suspension test (TST), or forced swim test (FST). Graphs are plotted as mean + SEM (n = 10 per group). Data were analyzed by multivariate ANOVA followed by independent samples t-test. *P < 0.05 compared to 0 mg/kg LPS group, ^(∗)0.05 > P > 0.1 compared to 0 mg/kg LPS group, ^# P < 0.05 compared to Temp1 (SIH) ^(#)0.05 > P > 0.1 compared to Temp1 (SIH).

Figure 5

Intraperitoneal injection of LPS caused a transient reduction in total volume intake and sucrose preference in the sucrose preference test. During the familiarization phase of the experiment (left), animals were familiarized to the experimental setup. On day (D) 1 and D3, mice were exposed to 2 bottles of water (W/W), while on D2 and D4 one bottle contained water and the other bottle was filled with a 1, 2, 5, or 10% sucrose solution (W/S). Voluntary consumption of water or sucrose was measured during a period of 24 h for up to 3 days after systemic LPS administration (D8–D10). Dashed lines indicate chance level for sucrose preference. Graphs are plotted as mean + SEM (n = 10 per group). Data were analyzed by repeated measures ANOVA followed by independent samples t-test. *P < 0.05 compared to 10% sucrose/vehicle group, ^# P < 0.05 compared to 10% sucrose/LPS group. W/W: water/water; W/S: water/sucrose.