Poly (I:C)-induced maternal immune activation modifies ventral hippocampal regulation of stress reactivity: prevention by environmental enrichment

Abstract

Environmental enrichment (EE) has been successfully implemented in human rehabilitation settings. However, the mechanisms underlying its success are not understood. Incorporating components of EE protocols into our animal models allows for the exploration of these mechanisms and their role in mitigation. Using a mouse model of maternal immune activation (MIA), the present study explored disruptions in social behavior and associated hypothalamic pituitary adrenal (HPA) axis functioning, and whether a supportive environment could prevent these effects. We show that prenatal immune activation of toll-like receptor 3, by the viral mimetic polyinosinic-polycytidylic acid (poly(I:C)), led to disrupted maternal care in that dams built poorer quality nests, an effect corrected by EE housing. Standard housed male and female MIA mice engaged in higher rates of repetitive rearing and had lower levels of social interaction, alongside sex-specific expression of several ventral hippocampal neural stress markers. Moreover, MIA males had delayed recovery of plasma corticosterone in response to a novel social encounter. Enrichment housing, likely mediated by improved maternal care, protected against these MIA-induced effects. We also evaluated c-Fos immunoreactivity associated with the novel social experience and found MIA to decrease neural activation in the dentate gyrus. Activation in the hypothalamus was blunted in EE housed animals, suggesting that the putative circuits modulating social behaviors may be different between standard and complex housing environments. These data demonstrate that augmentation of the environment supports parental care and offspring safety/security, which can offset effects of early health adversity by buffering HPA axis dysregulation. Our findings provide further evidence for the viability of EE interventions in maternal and pediatric settings.

Keywords: Autism; CamkIIa, Protein Kinase; Corticotropin releasing factor; Corticotropin releasing hormone receptor; Fetal programming; Glucocorticoid receptor; Hippocampus; Inflammation; Maternal behavior; Maternal care; Oxytocin; Poly I:C; Prefrontal cortex; Schizophrenia; Sex difference; Social approach; Social vigilance; Suprammamillary nucleus; Synaptic plasticity; Vasopressin; cFos.

Figure 1.. Experimental Timeline and Maternal Measures.

(A) Flow chart of experiment procedures. Pictures of the (B) environmental enrichment and (C) standard housing conditions. (D) Maternal body weight differences (grams) between gestational days (G) 11 to 12 and days 12 to 13, and (E) postnatal day 15 nest quality scores following maternal immune activation (MIA) and environmental enrichment housing. Data are expressed as mean ± SEM, n=12–14 dams per MIA and housing group. *p < 0.05, **p < 0.01, ***p <0.001, versus SD-saline; #p < 0.05, ##p < 0.01, ###p <0.001, versus EE-poly (I:C).

Figure 2.. The effects of maternal immune activation (MIA) and environmental enrichment on adult offspring social behavior.

(A-G) Represent data from the postnatal day (P) 70 social preference test. Social preference index for (A) male, (B) female and (C) both sexes combined. (D) Percent of time in the center and (E) distance traveled (mm) in the open field test for male and female offspring. (F) The duration (seconds) of time spent grooming and (G) the number of rears. (H-L) Represent data from the P85 social investigation test. The duration of time (seconds) spent directly investigating a novel conspecific for (H) male and (I) female offspring. Total time (seconds) that (J) males and (K) females spent in the interaction zone during the social investigation experience. (L) Total time (seconds) that male and female mice spent in social vigilance. Data are expressed as mean ± SEM, n = 9–13 litters represented per sex, MIA, and housing group. *p < 0.05, **p < 0.01, ***p <0.001, versus SD-saline; #p < 0.05, ##p < 0.01, ###p <0.001, versus EE-poly (I:C); ^ap < 0.05, ^aap < 0.01, ^aaap < 0.001, main effect of housing.

Figure 3.

Central c-Fos activation following a postnatal day (P) 85 social exposure in male and female offspring exposed to maternal immune activation (MIA) and environmental enrichment (EE). (A) Coronal section of the medial prefrontal cortex, including prelimbic (PL) and infralimbic (IL) regions. (B) Coronal section of the hippocampus and hypothalamus including CA1, CA2, CA3, dentate gyrus (DG), the supramammillary nucleus (SuM) and whole hypothalamus. Mean c-Fos (per mm²) in the (C) medial prefrontal cortex PL and IL regions, (D) hippocampus (CA1, CA2, CA3, DG), and (E) SuM and whole hypothalamus following a ten-minute exposure to a novel social conspecific. (F) Representative images of c-Fos immunostaining in the dentate gyrus (top) and hypothalamus, including SuM (bottom) of standard housed (SD)-saline, SD-poly (I:C), EE-saline and EE-poly (I:C) male and female offspring. Scale bar equals 200 μm. (G) Pearson correlations of c-Fos activation and time (seconds) spent in direct social investigation, for SD (top) and EE (bottom) mice. Correlated neural activity patterns computed from Pearson’s bivariate correlations of c-Fos positive cells between the hypothalamus (including SuM) and hippocampal regions for (H) SD-saline, (I) SD-poly (I:C), (J) EE-saline, and (K) EE- poly (I:C) mice. Mean ± SEM. There were no significant sex differences so male and female data were collapsed together; n=13–17 total per group. ^ap < 0.05, ^aap < 0.01, main effect of housing; ^bp < 0.05, ^bbp < 0.01, main effect of MIA; *p <0.05, **p<0.01, significant Pearson’s correlations

Figure 4.

Ventral hippocampal gene expression following maternal immune activation (MIA) and environmental enrichment housing. Male and female offspring levels of (A,B) Oxtr, (C,D) Crh, (E, F) Crhr1, (G, H) and Nr3c1 expressed as relative mRNA expression and (I, J) plasma corticosterone. Pearson correlations of ventral hippocampal relative mRNA expression and time (seconds) spent in (K-O) direct social investigation, (P-T) the interaction zone, and (U-Y) social vigilance on postnatal day 85. Gene expression data are expressed as Mean ± SEM, n=8 litters represented per sex, MIA, and housing group. *p < 0.05, **p < 0.01, ***p <0.001, versus SD-saline; #p < 0.05, ##p < 0.01, ###p <0.001, versus EE-poly (I:C).