Cognitive effects of early life exposure to PCBs in rats: Sex-specific behavioral, hormonal and neuromolecular mechanisms involving the brain dopamine system

Abstract

Endocrine-disrupting chemicals (EDCs) are environmental toxicants that disrupt hormonal and neurodevelopmental processes. Among these chemicals, polychlorinated biphenyls (PCBs) are particularly concerning due to their resistance to biodegradation and tendency to bioaccumulate. PCBs affect neurodevelopmental function and disrupt the brain's dopamine (DA) system, which is crucial for attentional, affective, and reward processing. These disruptions may contribute to the rising prevalence of DA-mediated neuropsychiatric disorders such as ADHD, depression, and substance use disorders. Notably, these behaviors are sexually dimorphic in part due to differences in sex hormones and their receptors, which are targets of estrogenic PCBs. Therefore, this study determined effects of early life PCB exposure on behaviors and neurochemistry related to potential disruption of dopaminergic signaling. Male and female Sprague Dawley rats were exposed to the PCB mixture Aroclor 1221 (A1221) or vehicle perinatally and then underwent a series of behavioral tests in adulthood, including the sucrose preference test to measure anhedonia, conditioned orienting to assess incentive-motivational phenotype, and attentional set-shifting to evaluate cognitive flexibility and response latency. Following these tests, rats were euthanized, and serum estradiol (E2), DA cells in the midbrain ventral tegmental area (VTA) and substantia nigra (SN), and gene expression from those combined midbrain nuclei were measured. Female rats exposed perinatally to A1221 exhibited decreased sucrose preference, and both male and female A1221 rats had reduced response latency in the attentional set-shifting task compared to vehicle counterparts. Conditioned orienting and serum estradiol (E2)were not affected in either sex; however, A1221-exposed rats of both sexes displayed higher TH+ cell numbers in the VTA and increased expression of dopamine receptor 1 (Drd1) in the combined midbrain nuclei. Additionally, E2 uniquely predicted behavioral outcomes and VTA DAergic cell numbers in A1221-exposed female rats, whereas DA signaling genes were predictive of behavioral outcomes in males. These data highlight sex-specific effects of A1221 on neuromolecular and behavioral phenotypes.

Keywords: Attention; Dopamine; Endocrine disruption; Endocrine-disrupting chemical (EDC); Neuromolecular; PCBs; Reward.

Fig. 1.

Sucrose preference test results. Points indicate values for individual rats, N = 20/sex/treatment. A. Mean tap water or sucrose solution consumed in grams (g) ± SEM. Female rats with A1221 exposure drank a non-significantly higher amount of tap water (left panel) compared to Veh controls (p = 0.07). There was no difference in sucrose solution consumption between treatment groups in either sex. B. Mean percent (%) preference for sucrose solution ± SEM. The dashed line indicates the 50 % preference point, above which rats exhibit increased preference for the sucrose solution. Female rats with A1221 exposure had a significantly lower preference for sucrose solution compared to Veh controls (p = 0.05). Male sucrose preference was not different between treatment groups.

Fig. 2.

Orienting response (OR) and foodcup (FC) scores over Pavlovian conditioning, N = 20/sex/treatment. A. Mean OR scores over conditioning in CS1 and CS2 ± SEM. In both female (left panel) and male rats (right panel), OR scores were higher at the end of conditioning (block 8) in CS1 compared to CS2 (p < 0.001 for both). B. Mean FC scores over conditioning in CS1 and CS2 ± SEM. In both sexes, FC scores were higher at the end of conditioning (block 8) in CS2 compared to CS1 (p < 0.001 for both). C. Mean CS1 OR scores over conditioning ± SEM between treatment groups. There were no effects of treatment in either sex on acquisition of ORs in CS1. D. Mean CS2 FC scores over conditioning ± SEM. There were no effects of treatment in either sex on acquisition of FC behavior in CS2.

Fig. 3.

Response measures in the attentional set-shifting task. Points indicate values for individual rats, shape indicates counterbalanced order of response requirements. N = 20/sex/treatment. A. Mean total trials to criterion ± SEM. In both female and male rats, the total trials required to reach criterion did not differ between treatment groups in the attentional set task (left panel) nor after the shift in response requirements (right panel). B. Mean errors before criterion ± SEM. In both female and male rats, erroneous responses prior to reaching criterion did not differ between treatment groups in the attentional set task (left panel) nor after the shift in response requirements (right panel). C. Mean latency to respond in seconds (s) ± SEM. A1221 exposure decreased response latency compared to Veh controls (p = 0.01).

Fig. 4.

Serum estradiol (E2; N = 20/sex/treatment) and tyrosine hydroxylase positive (TH+; N = 10/sex/treatment) cells in the midbrain Ventral Tegmental Area and Substantia Nigra. Points indicate values for each individual. A. Mean serum E2 (pg/ml) ± SEM. In all behaviorally characterized female and male rats (n = 20/sex/treatment), serum E2 did not differ between treatment groups. B. Mean TH+ cells in the Ventral Tegmental Area (VTA) ± SEM. In the subset of rats used for immunohistochemistry (n = 10/sex/treatment), TH+ cell numbers were significantly higher in the A1221 treatment group compared to Veh controls (p = 0.05). C. Mean TH+ cells in the Substantia Nigra (SN) ± SEM. In the subset of rats used for immunohistochemistry (n = 10/sex/treatment), TH+ cell numbers did not differ between treatment groups.

Fig. 5.

Representative micrographs of tyrosine hydroxylase (TH) immunohistochemical labeling of cell bodies and processes, with arrows indicating a TH+ cell body in each region. Scale bar (black) = 25 μm. A. TH+cells in the Ventral Tegmental Area (VTA). ST = stria terminalis, used as a biological landmark for VTA imaging. B. TH+ cells in the Substantia Nigra.

Fig. 6.

Expression of target genes in the midbrain (substantia nigra and ventral tegmental area combined). Points indicate individual scores. N = 10/sex/treatment. A. Mean fold change in estrogen receptor alpha (Esr1) ± SEM. Esr1 expression did not differ between treatment groups. B. Mean fold change in estrogen receptor beta (Esr2) ± SEM. Esr2 expression did not differ between treatment groups. C. Mean fold change in dopamine receptor 1 (Drd1) ± SEM. A1221 exposure increased Drd1 expression relative to Veh controls (p = 0.03). Note: For A1221, 2 datapoints (1 per sex) were above the y-axis cutoff and are not shown. D. Mean fold change in dopamine receptor 2 (Drd2) ± SEM. In female and male rats, Drd2 expression did not differ between treatment groups. E. Mean fold change in dopamine transport gene Slc6a3 ± SEM. Slc6a3 expression did not differ between treatment groups. F. Mean fold change in tyrosine hydroxylase encoding gene Th ± SEM. Th expression did not differ between treatment groups.

Fig. 7.

Scatterplots with regression lines showing significant relationships between outcomes variables. Points indicate individual scores, and lines indicate trends within treatment groups; all data are standardized to z-scores. R² and p-values are from the omnibus regression analysis, accounting for both main and interaction effects, are shown. A. Serum estradiol (E2; N = 20/treatment) explained 17.9 % (R²) of the variation in the trials required to reach criterion after the response requirement shift in female rats (p = 0.02). The slope of the regression lines was significantly different between A1221 females and Veh controls. B. Serum estradiol (E2) explained 24.6 % (R²) of the variation in errors before reaching criterion after the response requirement shift in female rats (p = 0.02). The slope of the regression lines was significantly different between A1221 females and Veh controls. C. In the subset of female rats used for IHC (N = 10/treatment group), serum estradiol (E2) explained 24.6 % (R²) of the variation in Ventral Tegmental Area (VTA) dopamine (DA) cells (i.e., TH+ cells; p = 0.02). The slope of the regression lines was significantly different between A1221 females and Veh controls. D. In the subset of male rats used for PCR (N = 10/treatment group), expression of dopamine receptor 1 (Drd1) from the combined midbrain regions explained 39.1 % (R²) of the variation in preference for sucrose solution (p = 0.01). The slope of the regression lines was significantly different between A1221 males and Veh controls. The arrow indicates a potential outlier point in the regression model; when removed, the model and interaction term remained significant (p = 0.02 and p = 0.01, respectively; not shown). E. Expression of dopamine receptor 2 (Drd2) explained 39.1 % (R²) of the variation in response latency after the requirement shift in the attentional set-shifting task (p = 0.02). The slope of the regression lines was significantly different between A1221 males and Veh controls. F. Expression of dopamine transport gene Slc6a3 explained 24.6 % (R²) of the variation in response latency after the requirement shift in the attentional set-shifting task (p = 0.05). The slope of the regression lines was significantly different between A1221 males and Veh controls. F. Expression of dopamine transport gene Slc6a3 explained 24.6 % (R²) of the variation in response latency after the requirement shift in the attentional set-shifting task (p = 0.05). The slope of the regression lines was significantly different between A1221 males and Veh controls.