Sex-specific effects of developmental exposure to polychlorinated biphenyls on neuroimmune and dopaminergic endpoints in adolescent rats

Abstract

Exposure to environmental contaminants early in life can have long lasting consequences for physiological function. Polychlorinated biphenyls (PCBs) are a group of ubiquitous contaminants that perturb endocrine signaling and have been associated with altered immune function in children. In this study, we examined the effects of developmental exposure to PCBs on neuroimmune responses to an inflammatory challenge during adolescence. Sprague Dawley rat dams were exposed to a PCB mixture (Aroclor 1242, 1248, 1254, 1:1:1, 20 μg/kg/day) or oil control throughout pregnancy, and adolescent male and female offspring were injected with lipopolysaccharide (LPS, 50 μg/kg, ip) or saline control prior to euthanasia. Gene expression profiling was conducted in the hypothalamus, prefrontal cortex, striatum, and midbrain. In the hypothalamus, PCBs increased expression of genes involved in neuroimmune function, including those within the nuclear factor kappa b (NF-κB) complex, independent of LPS challenge. PCB exposure also increased expression of receptors for dopamine, serotonin, and estrogen in this region. In contrast, in the prefrontal cortex, PCB exposure blunted or induced irregular neuroimmune gene expression responses to LPS challenge. Moreover, neither PCB nor LPS exposure altered expression of neurotransmitter receptors throughout the mesocorticolimbic circuit. Almost all effects were present in males but not females, in agreement with the idea that male neuroimmune cells are more sensitive to perturbation and emphasizing the importance of studying both male and female subjects. Given that altered neuroimmune signaling has been implicated in mental health and substance abuse disorders that often begin during adolescence, these results highlight neuroimmune processes as another mechanism by which early life PCBs can alter brain function later in life.

Keywords: Dopamine; Endocrine disrupting compound; Estradiol; NF-κB; TLR4.

Figure 1:. Experimental timeline.

Pregnant dams were orally exposed to PCBs or oil control, throughout gestation (embryonic day E1 – E22/23). The day after birth (P1), up to four pups per litter were used for a previous study (Bell et al 2018), and then the litter was culled to four male and four female pups. Per litter, two males and two females were randomly assigned for use during adolescence (current study) or during adulthood (in a companion study, data not shown). Between P40–42, one male and one female were randomly assigned to receive an immune challenge (lipopolysaccharide, LPS, 50 ug/kg, i.p.) or saline vehicle control 2.5 hours prior to tissue collection.

Figure 2:. Male animals exposed to PCBs had significantly greater body weights.

This effect was present from P28 – 35. Data are presented as mean body weight values ± SEM across time, and data for P28 are shown in the top left inset. Final n per group are shown within insert bar graph and represent final group counts for all days analyzed. Significant effects (p < 0.05) are noted (*).

Figure 3:. Animals exposed to PCBs had greater expression of immune-related genes in the hypothalamus.

In response to PCB exposure, females had greater expression of Ikbkb (A) and males had greater expression of Rela (B), Nfkb1 (C), Itgam (D), and Tgfb2 (E). In both sexes, PCB exposure did not affect the relative expression of Tlr4. LPS also increased expression of Nfkb1 (B) and Rela (C) Data are presented as mean values ± SEM with final n per group, after removing outliers and samples that failed to amplify, shown within bars. Significant effects (*p < 0.05) are noted within sex.

Figure 4:. Males exposed to PCBs had greater expression of neuromodulator receptors in the hypothalamus.

In response to PCB exposure, males had greater expression of Esr2 (A), Htr2a (B), Drdla (C), and Drd2 (D). In both sexes, PCB exposure did not change the relative expression of Th. Data are presented as mean values ± SEM with final n per group, after removing outliers and samples that failed to amplify, shown within bars. Significant effects (**p < 0.01) are noted within sex.

Figure 5:. PCB exposure altered responses to LPS in male prefrontal cortex.

In response to PCB and LPS exposure, males showed a change in relative expression of Ikbkb (A), Rela (C), and Tlr4 (D) but not Nfkb1 (B). Data are presented as mean values ± SEM with final n per group, after removing outliers and samples that failed to amplify, shown within bars. Significant effects (*p < 0.05) are noted within sex.

Figure 6:. LPS increased concentrations of serum corticosterone in both males and females, independent of PCB exposure.

Data are presented as mean values ± SEM with final n per group, after outlier removal, shown within bars. Significant effects (**p < 0.01) are noted within sex.

Figure 7:. LPS increased concentrations of serum cytokines, IL1b (A), IL10 (B), IL6 (C), and TNF (D), in both males and females independent of PCB exposure.

In females exposed to PCBs, the increase in IL1b is blunted. Each group included 7–9 samples, but many were below detection limits (BDL). Because of this, and the high variability within groups, data are presented as mean values ± SEM, with detectable samples shown as overlaid data points. Significant effects (*p < 0.05) are noted within sex.