Early life stress leads to developmental and sex selective effects on performance in a novel object placement task

Abstract

Disruptions in early life care, including neglect, extreme poverty, and trauma, influence neural development and increase the risk for and severity of pathology. Significant sex disparities have been identified for affective pathology, with females having an increased risk of developing anxiety and depressive disorder. However, the effects of early life stress (ELS) on cognitive development have not been as well characterized, especially in reference to sex specific impacts of ELS on cognitive abilities over development. In mice, fragmented maternal care resulting from maternal bedding restriction, was used to induce ELS. The development of spatial abilities were tracked using a novel object placement (NOP) task at several different ages across early development (P21, P28, P38, P50, and P75). Male mice exposed to ELS showed significant impairments in the NOP task compared with control reared mice at all ages tested. In female mice, ELS led to impaired NOP performance immediately following weaning (P21) and during peri-adolescence (P38), but these effects did not persist into early adulthood. Prior work has implicated impaired hippocampus neurogenesis as a possible mediator of negative outcomes in ELS males. In the hippocampus of behaviorally naïve animals there was a significant decrease in expression of Ki-67 (proliferative marker) and doublecortin (DCX-immature cell marker) as mice aged, and a more rapid developmental decline in these markers in ELS reared mice. However, the effect of ELS dissipated by P28 and no main effect of sex were observed. Together these results indicate that ELS impacts the development of spatial abilities in both male and female mice and that these effects are more profound and lasting in males.

Fig. 1

ELS effects on hormonal, somatic, and motor development. Data was collected for control and ELS reared male and female mice across development. A) Mean AM basal serum corticosterone levels were higher in female mice compared with males and ELS led to increased basal corticosterone in both sexes. B) mean weight of females were lower than males, with all animals gaining weight across the study period. On average ELS reared mice were 17% smaller than control and ELS mice. C) No effect of sex or treatment was found for total distance traveled in the open field at P20. D) No effect of sex or treatment was found for % center time in the open field. Main effects of age, treatment and sex are identified with (*). Post-hoc ANOVA at a given age are identified with (^#). ***p < 0.001, ^#p < 0.05. Data are presented as mean (+/− SEM).

Fig. 2

ELS has developmental and sex selective effects on NOP performance. T2 fraction for mice during the test phase of the NOP task. T2 fraction was calculated as the ratio (novel object investigation/(novel + familiar object investigation)*100. There was no difference in performance between control male and female mice. (A) For male mice, a significant main effect fo treatment was found, with ELS mice failing to perform above chance at all ages tested. (B) For female mice, ELS led to impaired performance at P21 and P38 but not other developmental time points. *denote significant main effect (ANOVA). ^#denotes significant single sample t-test, comparing T2 fraction to chance (50). ^adenotes an effect approaching but failing to reach significance (p < 0.10). Data represent the group mean (+/-SEM).

Fig. 3

ELS does not alter motor activity in the NOP task. We observed a significant effect of age, but no effects of sex or treatment on locomotor activity during T1 exploration phase for either (A) male or (B) female mice. During the T2 recognition phase, we also observed a significant effect of age, but no effects of sex or treatment on locomotor activity for (C) male or (D) female mice. *alpha was set at (p < 0.05). Data represent the group mean (+/-SEM).

Fig. 4

ELS does not alter object exploration during the NOP task. For object exploration during the T1 exploration phase of testing we observed a significant effect of age for (A) male and (B) female mice, but no effects of sex or treatment or interactions. During the T2 recognition phase of testing, we observed a main effect of sex, but no effect of treatment, age, or interactions for (C) male and (D) female mice. *alpha was set at (p < 0.05). Data represent the group mean (+/-SEM).

Fig. 5

Degree of T1 investigation is not correlated with performance during the T2 phase of the NOP task. The relationship between performance during the T2 recognition phase of testing and duration of object exploration during the T1 exploration phase of testing is plotted for each individual animal. Plots at R² are included for animals at A) P21 B) P28 C) P38 D) P50 and E) P75. No significant relationship between T1 exploration measures and performance during the T2 recognition phase of the task at any age. Based upon these data, greater investigation of the objects during the T1 exploration phase does not promote better performance on during the T2 recognition phase.

Fig. 6

ELS alters the developmental expression of markers of neuronal proliferation and differentiation similarly in male and female hippocampus. Gene expression for Ki-67 in (a) male and (b) female whole hippocampus as well as doublecortin (DCX) in (c) male and (d) female whole hippocampus. Gene expression was assessed by realtime qPCR and calculated relative to 18S. We observed a significant decrease in expression over development, and observed lower levels of expression in ELS reared hippocampus at early developmental ages for both sexes and both markers, but no significant effects of sex or treatment beyond P28. *significant main effects ^#significant post-hoc (Tukey LSD). Alpha was set at (p < 0.05). Data represent the group mean (+/−SEM).