Behavioral and genetic evaluation of the effect of prenatal stress on functional cerebral asymmetries in rats

Functional cerebral asymmetry (FCA) refers to the phenomenon in which the brain hemispheres specialize in regulating specific functions. In the past, it was assumed that FCAs are driven by genetic factors and remain constant throughout the lifespan once developed. Recent studies have demonstrated that FCAs have some degree of plasticity and are susceptible to environmental influences, potentially can be changed throughout an individual's lifespan. A growing body of evidence indicates the idea that environmental factors, such as stress, are associated with both functional and structural changes in cerebral asymmetries. Moreover, previous studies have demonstrated that applying prenatal stress induces enduring alterations in behavioural and neurochemical asymmetries in rat offspring, which persist into adulthood. The present study hypothesized that prenatal stress exposure alters the FCAs in rats. The primary aim of this study was to examine whether there are behavioural and molecular alterations induced by prenatal stress during various postnatal developmental phases in offspring. In this context, it was aimed to elucidate the genetic basis of cerebral lateralisation and to understand how prenatal stress affects brain organization at the gene level through microarray-based gene expression analysis. To this end, RNAs obtained from the primary and secondary motor cortices of the left and right hemispheres of the rat brains were used as a biological sample. Another object of this study was to examine sex-dependent differences in behavioural asymmetry and genetic regulation in offspring exposed to prenatal stress. This study also aimed to investigate the impact of prenatal stress on the maternal care behaviour of the dams. The subjects of the present study comprised 16 timed-pregnant Sprague-Dawley rats and their offspring (N= 131). A total of 16 pregnant rats were included in the study, with 8 rats assigned to the stress group (SG) and the remaining 8 to the control group (CG). The restriction stress protocol was applied to the dams for 7 days, covering gestation days 15-21 with a duration of 2 hours per day. Maternal care behaviours of the dams were assessed to evaluate the effect of stress exposure during gestation, and no differences were found between stress and control groups. The dams were administered the Elevated Plus Maze (EPM) test to evaluate the effectiveness of the movement restriction stress protocol. EPM test results showed that dams exposed to restriction stress showed fewer exploratory behaviours and more anxiety-related behaviours, indicating that the restriction stress protocol worked successfully. After weaning, the EPM test was applied to the offspring to measure anxiety-related behaviours, and different motor laterality tests were applied to measure FCAs. The results of the EPM test showed that in the PD60 age group, the offspring in the SG spent more time in the closed arms, while the pups in the CG spent more time in the open arms. This indicates that prenatal stress even has a long-lasting effect on offspring. The findings of this study revealed that prenatal stress exposure does not influence the general turning asymmetry or head-turning asymmetry in rats. In the present study, the Collins paw preference test and Pasta Matrix Reaching tests were used to assess the paw preference of the rats. The findings of these motor laterality tests showed that the direction of the first paw intervention of the rats is a good indicator of the overall paw preference of the individuals for the direction. The findings also showed that the paw preference of the rats is task dependent. The main finding of this study indicates that prenatal stress alters functional cerebral asymmetries in rats. Additionally, an intriguing finding from the Collins test in the present study is that prenatal stress induces a population- level leftward bias in the paw preference of rats, suggesting that stress exposure cause to right hemispheric activation. Microarray gene expression analysis was conducted on female individuals belonging to the PD60 group, exhibiting a more pronounced impact of prenatal stress. According to microarray-based gene expression analysis, the number of differentially expressed genes in the right hemisphere was found much higher than in the left hemisphere, indicating that prenatal stress has a greater impact on the right hemisphere at the gene expression level. According to microarray results, the FOXE1 and POLM genes are the most differently expressed genes with the greatest fold changes between the right hemispheres of female offspring in SG and CG. Considering the differential modulation of the POLM gene in reaction to prenatal stress and its association with behavioural laterality, specifically in the right hemisphere, suggests that the POLM gene holds promise as a potential candidate gene for future research studies investigating the link between stress and lateralisation. Overall, these results prove that prenatal stress exposure can change behavioral and hemispheric asymmetries in rats. Keywords: Behavioural laterality, FOXE1, Microarray, POLM, Prenatal stress