Inflammation early in life is a vulnerability factor for emotional behavior at adolescence and for lipopolysaccharide-induced spatial memory and neurogenesis alteration at adulthood

Abstract

Background: The postnatal period is a critical time window during which inflammatory events have significant and enduring effects on the brain, and as a consequence, induce alterations of emotional behavior and/or cognition later in life. However, the long-term effect of neonatal inflammation on behavior during adolescence, a sensitive period for the development of neurodevelopmental psychiatric disorders, has been little studied. In this study, we examined whether an early-life inflammatory challenge could alter emotional behaviors and spatial memory at adolescence and adulthood and whether stress axis activity, inflammatory response and neurogenesis were affected.

Methods: Lipopolysaccharide (LPS, 100 μg/kg) was administered to mice on postnatal day (PND) 14 and cytokine expression was measured in the plasma and in brain structures 3 hours later. Anxiety-like and depressive-like behavior (measured in the novelty-suppressed feeding test and the forced swim test, respectively) and spatial memory (Y-maze test) were measured at adolescence (PND30) and adulthood (PND90). Hypothalamic-pituitary-adrenal (HPA) axis activity (plasma corticosterone and glucocorticoid receptors in the hippocampus and prefrontal cortex) was measured at adulthood. In addition, the impact of a novel adult LPS challenge (100 μ/kg) was measured on spatial memory (Y-maze test), neurogenesis (doublecortin-positive cell numbers in the hippocampus) and plasma cytokine expression.

Results: First, we show in PND14 pups that a peripheral administration of LPS induced the expression of pro- and anti-inflammatory cytokines in the plasma and brain structures that were studied 3 hours after administration. Anxiety-like behavior was altered in adolescent, but not in adult, mice, whereas depressive-like behavior was spared at adolescence and increased at adulthood. This was accompanied by a decreased phosphorylation of the glucocorticoid receptor in the prefrontal cortex, with no effect on corticosterone levels. Second, neonatal LPS treatment had no effect on spatial memory in adolescence and adulthood. However, a second challenge of LPS in adulthood impaired spatial memory performance and neurogenesis and increased circulating levels of CCL2.

Conclusions: Our study shows for the first time, in mice, that a peripheral LPS treatment at PND14 differentially alters emotional behaviors, but not spatial memory, at adolescence and adulthood. The behavioral effect of LPS at PND14 could be attributed to HPA axis deregulation and neurogenesis impairment.

Figure 1

Schematic timeline of the study design and experimental protocols.

Figure 2

Lipopolysaccharide (LPS) increases cytokines expression in plasma and brain structures of postnatal day 14 (PND14) mice. Plasma concentration (pg/ml) of cytokines was measured by BioPlex 3 h after intraperitoneal (i.p.) administration of saline or LPS (100 μg/kg) in mice (A). Cytokine mRNA expression was measured by real-time rt-PCR in the hypothalamus (B), hippocampus (C), prefrontal cortex (D) and amygdala (E). Data are expressed as mean ± SEM, *P < .05, **P < .01, ***P < .001, for NaCl compared to LPS, n = 20.

Figure 3

Postnatal day 14 (PND14) neonatal lipopolysaccharide (LPS) differentially alters anxiety-like behavior, depressive-like behavior and spatial memory at adolescence (PND30) and adulthood (PND90). Anxiety-like behavior was evaluated in the novelty-suppressed feeding test (NSFT) by measuring the latency (sec) of first food consumption in the center of the arena (n = 20). Depressive-like behavior was evaluated in the forced swim test (FST) by measuring the swimming occurrence (number) during 5 min (n = 20). Data are expressed as mean ± SEM. P <0.05. Spatial memory was evaluated in the Y-maze test by measuring the time spent exploring the novel and the familiar arms over a 5-min test and after a 30-min retention delay (n = 20). Data are expressed as mean ± SEM. *P <0.05, ***P <0.001, for familiar arm compared to novel arm.

Figure 4

Postnatal day 14 (PND14) neonatal lipopolysaccharide (LPS) decreases the phosphorylated glucocorticoid receptor/glucocorticoid receptor ratio in the prefrontal cortex at adulthood. Representative immunoblots corresponding to the phosphorylated form of glucocorticoid receptor (PGR) and total glucocorticoid receptor (GR) in the prefrontal cortex (A) and the hippocampus (B) of adult mice receiving either NaCl or LPS at PND14 are presented. Immunoblots were quantified and the ratios for PGR/GR (left graph panels) or GR/actin were calculated (right graph panels). Data are expressed as mean ± SEM (n = 10/group). *P <0.05, for NaCl compared to LPS. Plasma corticosterone levels were measured by radioimmunoassay (RIA) in the plasma (C). Data are expressed as mean ± SEM (n = 10/group). *P <0.05, for NaCl compared to LPS.

Figure 5

A lipopolysaccharide (LPS) challenge at adulthood impairs spatial memory in postnatal day 14 (PND14) neonatal LPS mice. Spatial memory performance was evaluated in the Y-maze test by measuring the time spent exploring the novel and the familiar arms. Measures were performed 24 h after intraperitoneal (i.p.) administration of saline or lipopolysaccharide (LPS, 100 μg/kg) in adult mice. They were assessed over a 5-min test and after a 30-min retention intertrial interval (ITI). Data are expressed as means ± SEM (n = 15/group). *P <0.05, **P <0.01, for familiar arm compared to novel arm.

Figure 6

A lipopolysaccharide (LPS) challenge at adulthood reduces the number of doublecortin (DCX)-positive cells in the dentate gyrus of the hippocampus of postnatal day 14 (PND14) neonatal LPS mice. Neurogenesis was evaluated by quantifying the number of DCX-positive cells in the dentate gyrus of the hippocampus. Data are expressed as means ± SEM (n = 8/group). *P <0.05, for the NaCl-NaCl group compared to the LPS-LPS group.

Figure 7

A lipopolysaccharide (LPS) challenge at adulthood increases CCL2 in the plasma of postnatal day 14 (PND14) neonatal LPS mice. CCL2 was measured in the plasma of adult mice by BioPlex. Data are expressed as means ± SEM (n = 7/group). *P = 0.05, for the NaCl-NaCl group compared to the LPS-LPS group.