Late prenatal immune activation in mice leads to behavioral and neurochemical abnormalities relevant to the negative symptoms of schizophrenia

Abstract

Based on the human epidemiological association between prenatal infection and higher risk of schizophrenia, a number of animal models have been established to explore the long-term brain and behavioral consequences of prenatal immune challenge. Accumulating evidence suggests that the vulnerability to specific forms of schizophrenia-related abnormalities is critically influenced by the precise timing of the prenatal immunological insult. In the present study, we tested the hypothesis whether late prenatal immune challenge in mice may induce long-term behavioral and neurochemical dysfunctions primarily associated with the negative symptoms of schizophrenia. We found that prenatal exposure to the viral mimic polyriboinosinic-polyribocytidilic acid (Poly-I:C; 5 mg/kg, i.v.) on gestation day (GD) 17 led to significant deficits in social interaction, anhedonic behavior, and alterations in the locomotor and stereotyped behavioral responses to acute apomorphine (APO) treatment in both male and female offspring. In addition, male but not female offspring born to immune challenged mothers displayed behavioral/cognitive inflexibility as indexed by the presence of an abnormally enhanced latent inhibition (LI) effect. Prenatal immune activation in late gestation also led to numerous, partly sex-specific changes in basal neurotransmitter levels, including reduced dopamine (DA) and glutamate contents in the prefrontal cortex and hippocampus, as well as reduced γ-aminobutyric acid (GABA) and glycine contents in the hippocampus and prefrontal cortex, respectively. The constellation of behavioral and neurochemical abnormalities emerging after late prenatal Poly-I:C exposure in mice leads us to conclude that this immune-based experimental model provides a powerful neurodevelopmental animal model especially for (but not limited to) the negative symptoms of schizophrenia.

Figure 1

Social interaction deficits following late prenatal immune challenge. Social interaction was assessed by analyzing the relative exploration time between an unfamiliar congenic mouse (‘mouse') and an inanimate dummy object (‘dummy'). The line plots depict the relative exploration time as a function of successive 1-min bins, and the bar plot shows the mean exploration time per bin. Control offspring born to vehicle-treated mothers displayed a significant preference towards the unfamiliar congenic mouse, indicating intact social interaction. On the other hand, offspring exposed to prenatal Poly-I:C treatment in late gestation displayed no such preference. ^*P<0.05, based on restricted ANOVAs. All values are means±SEM.

Figure 2

Presence of anhedonic behavior following late prenatal immune challenge. Anhedonic behavior was assessed using a 2-day (48 h) sucrose preference test, in which animals had free choice between regular tap water and a 0.5% sucrose solution. (a) The graph depicts sucrose preference as indexed by a percentage score (sucrose consumption/(total liquid consumption) × 100%) obtained on two consecutive test days (day 1 and 2). Adult offspring exposed to prenatal Poly-I:C treatment in late gestation displayed a significant reduction in the sucrose preference index compared with adult control offspring. ^**P<0.01, signifies the main effect of prenatal treatment in the corresponding RM-ANOVA. (b) The graph shows the total fluid (water+sucrose solution) consumption on the two consecutive test days. Prenatal Poly-I:C treatment did not affect total fluid intake across the two test days. All values are means±SEM.

Figure 3

Presence of abnormally enhanced LI following late prenatal immune challenge. The LI effect was studied in a two-way active avoidance procedure, in which pre-exposed (PE) subjects were presented with a low number of the CS before conditioning, whereas NPE subjects were not pre-exposed to the CS. The graph depicts the response latency as a function of blocks of 10 trials in male and female offspring. Male but not female offspring exposed to prenatal Poly-I:C treatment in late gestation displayed a significant LI effect (ie, reduced response latency in PE relative to NPE subjects) under parametric conditions, in which control offspring did not display LI. ^*P<0.05, signifies the significant main effect of pre-exposure in the corresponding ANOVA restricted to male Poly-I:C offspring. All values are means±SEM.

Figure 4

Late prenatal immune activation leads to altered locomotor and stereotyped behavioral responses to systemic APO treatment. (a) The graph depicts locomotor activity as indexed by the horizontal distance moved following systemic ascorbic acid (VitC, vehicle) treatment and following systemic APO (5 mg/kg, s.c.) treatment. Horizontal distance moved is plotted as a function of 5-min bins. Adult offspring subjected to prenatal Poly-I:C exposure in late gestation displayed an overall significant increase in the horizontal distance moved following APO treatment relative to APO-treated offspring born to control mothers. ^*P<0.05, signifies the significant main effect of prenatal treatment in the corresponding RM-ANOVA. (b) The graph shows the time spent climbing on successive sampling intervals (1-min observation period every 5 min) following VitC and APO treatment. Adult Poly-I:C offspring displayed a significant increase in the time spent climbing specifically during the first observation period compared with APO-treated control offspring. ^*P<0.05, signifies the significant main effect of prenatal treatment in the corresponding ANOVA restricted to the first observation period following APO treatment. (c) The graph depicts the time spent leaning on successive sampling intervals (1-min observation period every 5 min) following VitC and APO treatment. Adult Poly-I:C offspring displayed a significant decrease in the time spent leaning specifically during the first observation period compared with APO-treated control offspring. ^*P<0.05, signifies the significant main effect of prenatal treatment in the corresponding ANOVA restricted to the first observation period following APO treatment. (d) The graph depicts the time spent sniffing on successive sampling intervals (1-min observation period every 5 min) following VitC and APO treatment. There were no significant group differences in the time spent sniffing neither in the initial VitC phase nor in the subsequent APO phase. All values are means±SEM.

Figure 5

Effects of late prenatal immune challenge on basal dopamine (DA) and serotonin (5-HT) levels. Levels of DA and 5-HT were determined in postmortem brain tissue using high-performance liquid chromatography (HPLC). Monoamine contents were measured in the mPFC, AMY, CPu, NAc, dHPC, and vHPC. All monoamine levels are expressed as ng per mg fresh tissue weight. (a) DA contents in adult control and Poly-I:C offspring. ^*P<0.05, based on ANOVA of DA content in the corresponding brain area. The inlets provide DA levels in the mPFC and vHPC at higher magnification. (b) 5-HT contents in adult control and Poly-I:C offspring. Symbol (§) signifies the presence of sex-dependent effects, as further depicted by the inlets showing 5-HT levels in the AMY and NAc of male (M) and female (F) offspring born to Poly-I:C-exposed mothers or control mothers. ^*P<0.05, based on the ANOVA restricted to females. All values are means±SEM.

Figure 6

Effects of late prenatal immune challenge on basal levels of excitatory amino acids. Levels of the excitatory amino acids glutamate and aspartate were determined in postmortem brain tissue using HPLC. The contents of these excitatory amino acids were measured in the mPFC, AMY, CPu, NAc, dHPC, and vHPC. All levels are expressed as ng per mg fresh tissue weight. (a) Glutamate contents in adult control and Poly-I:C offspring. Symbol (§) signifies the presence of a sex-dependent effect in the mPFC, as further depicted by the inlets showing glutamate levels in the mPFC of male (M) and female (F) offspring born to Poly-I:C-exposed mothers or control mothers. ^**P<0.01, based on the ANOVA restricted to males. ^##P<0.01, based on ANOVA in males and females. (b) Aspartate contents in adult control and Poly-I:C offspring. Symbol (§) signifies the presence of a sex-dependent effect in the mPFC, as further depicted by the inlets showing aspartate levels in the mPFC of male (M) and female (F) offspring born to Poly-I:C-exposed mothers or control mothers. ^***P<0.001, based on the ANOVA restricted to males. ^##P<0.01, based on ANOVA of dHPC contents in males and females. All values are means±SEM.

Figure 7

Effects of late prenatal immune challenge on basal levels of inhibitory amino acids. Levels of the inhibitory amino acids GABA and taurine were determined in postmortem brain tissue using HPLC. The contents of these inhibitory amino acids were measured in the mPFC, AMY, CPu, NAc, dHPC, and vHPC. All levels are expressed as ng per mg fresh tissue weight. (a) GABA contents in adult control and Poly-I:C offspring. Symbol (§) signifies the presence of a sex-dependent effect in the dHPC, as further depicted by the inlets showing GABA levels in the dHPC of male (M) and female (F) offspring born to Poly-I:C-exposed mothers or control mothers. ^*P<0.05, based on the ANOVA restricted to females. (b) Taurine contents in adult control and Poly-I:C offspring. Symbol (§) signifies the presence of sex-dependent effects, as further depicted by the inlets showing GABA levels in the mPFC and dHPC of male (M) and female (F) offspring born to Poly-I:C-exposed mothers or control mothers. ^**P<0.01, based on the ANOVA restricted to males; ^*P<0.01, based on the ANOVA restricted to females. All values are means±SEM.