Impact of mothers’ experience and early‐life stress on aggression and cognition in adult male mice

The role of gene-environment interactions in social dysfunction: Focus on preclinical evidence from mouse studies

Human and animal research has demonstrated that genetic and environmental factors can strongly modulate behavioral function, including the expression of social behaviors and their dysfunctionalities. Several genes have been linked to pathologies characterized by alterations in social behaviors, e.g., aggressive/antisocial personality disorder (ASPD), or autism spectrum disorder (ASD). Environmental stimulation (e.g., physical exercise, environmental enrichment) or adversity (e.g., chronic stress, social isolation) may respectively improve or impair social interactions. While the independent contribution of genetic and environmental factors to social behaviors has been assessed in a variety of human and animal studies, the impact of their interactive effects on social functions has been less extensively investigated. Genetic mutations and environmental changes can indeed influence each other through complex mutual effects, e.g., inducing synergistic, antagonistic or interactive behavioral outcomes. This complexity is difficult to be disentangled in human populations, thus encouraging studies in animal models, especially in the mouse species which is the most suitable for genetic manipulations. Here we review the available preclinical evidence on the impact of gene-environment interactions on social behaviors and their dysfunction, focusing on studies in laboratory mice. We included findings combining naturally occurring mutations, selectively bred or transgenic mice with multiple environmental manipulations, including positive (environmental enrichment, physical exercise) and aversive (social isolation, maternal separation, and stress) experiences. The impact of these results is critically discussed in terms of their generalizability across mouse models and social tests, as well as their implications for human studies on social dysfunction.

Maternal separation early in life induces excessive activity of the central amygdala related to abnormal aggression

Epidemiological studies have indicated that child maltreatment, such as neglect, is a risk factor of escalated aggression, potentially leading to delinquency and violent crime in the future. However, little is known about the mechanisms by which an early adverse environment may later cause violent behavior. In this study, we aimed to thoroughly examine the association between aggression against conspecific animals and the activity of amygdala subnuclei using the maternal separation (MS) model, which is a common model of early life stress. In the MS group, pups of Sprague-Dawley rats were separated from their dam during postnatal days 2-20 (twice a day, 3 h each). We only included 9-week-old male offspring for each analysis and compared the MS group with the mother-reared control group; both groups were raised by the same dam during postnatal days 2-20. The results revealed that the MS group exhibited higher aggression and excessive activity of only the central amygdala (CeA) among the amygdala subnuclei during the aggressive behavior test. Moreover, a significant positive correlation was observed between higher aggression and CeA activation. While CeA activity is known to be involved in hunting behavior for prey, some previous studies have also indicated a relationship between CeA and intraspecific aggression. It remains unclear, however, whether excessive CeA activity directly induces intraspecific aggression. Therefore, we stimulated the CeA using optogenetics with 8-week-old rats to clarify the relationship between intraspecific aggression and CeA activity. Notably, CeA activation resulted in higher aggression, even when the opponent was a conspecific animal. In particular, bilateral CeA activation resulted in more severe displays of aggressive behavior than necessary, such as biting a surrendered opponent. These findings suggest that an adverse environment during early development intensifies aggression through excessive CeA activation, which can increase the risk of escalating to violent behavior in the future.

Evaluation of changes in the cognitive function of adult cynomolgus monkeys under stress induced by audio-visual stimulation by applying modified finger maze test

Stress in life is ubiquitous and unavoidable. Prolonged exposure to severe stress can lead to physical intolerance and impair cognitive function. Non-human primates are considered to be the best animal model for studying cognitive function, especially memory and attention. The finger maze test, with the advantages of short training time and lower cost, is recommended to evaluate learning and memory in non-human primates. In this study, we modified the finger maze test method to evaluate the cognitive function of single-housed cynomolgus monkeys. The flexibility and attention of cynomolgus monkeys were assessed by performing the complex task test and the stranger intrusion interference test, respectively, which increased the difficulty of obtaining rewards, and the ability of long-term memory was also evaluated by the memory test. Furthermore, the changes in cognitive function of the cynomolgus monkeys were tested by using the finger maze test after audio-visual stimulation, and the changes in the cortisol levels during stimulation were also analyzed. We found that, after completing the learning test, there was no significant decrease in their success rate when monkeys processed multitasks at the same time. In the stranger intrusion interference test, all subjects were distracted, but the accuracy did not decrease. The monkeys completed the memory tests in the 1st and 2nd months after the learning tests, with a high success rate. However, the success rate decreased significantly at the end of the 4th month. During audio-visual stimulation, the plasma cortisol level significantly increased in the first 2 months and was maintained at a high level thereafter. One month after audio-visual stimulation, the accuracy of the memory test was significantly reduced, and the total time of distraction was significantly prolonged. In conclusion, chronic audio-visual stimulation can increase blood cortisol levels and impair cognitive function. The modified finger maze test can evaluate many aspects of cognitive function and assess the changes in the cognitive function of adult cynomolgus monkeys under stress.

High-resolution MRI data of the brain of C57BL/6J and BTBR mice in three anatomical views

The research on strain-, sex-, and stress-specific differences in structural and functional connectivity of the brain is important for elucidating various behavioral features and etiologies of psychiatric disorders. Socially impaired BTBR mice are considered a model of autism spectrum disorders. Here we present high-resolution magnetic resonance imaging data from the brain of 89 adolescent mice (C57BL/6J and BTBR) in axial, sagittal, and coronal views. The study [1] includes both females and males differed in early-life experience (normally reared or subjected to prolonged maternal separation: 3 h daily from postnatal day 2 to 15). The MRI data were obtained on a horizontal tomograph Biospec 117/16 instrument with a magnetic field strength of 11.7 T. Thus, multislice Turbo RARE T₂-weighted images of the brain were captured in eight groups of mice. Altogether, these data allow to evaluate strain-, sex-, and stress-specific alterations in the volumes of various brain structures and to better understand the relation between brain structural differences and behavioral abnormalities.

Neonatal procedural pain affects state, but not trait anxiety behavior in adult rats

The influence of neonatal experiences upon later-life affective behavior is increasingly recognized, but the reported effects on anxiety are often contradictory. The observed effect may depend upon the type of anxiety (state or trait) affected. The current study aims to investigate whether neonatal repetitive needle pricking alters anxiety behavior in adulthood, by assessing both state and trait anxiety in rats. Sprague-Dawley rat pups received four unilateral needle pricks per day, while controls received four tactile stimuli or were left completely undisturbed during the first postnatal week. Mechanical sensitivity was assessed in the neonatal phase and throughout the development. State anxiety was assessed in the open field test and trait anxiety in the elevated zero maze. The results show that repetitive needle pricking leads to acute mechanical hypersensitivity, but does not affect baseline mechanical sensitivity throughout development. In adulthood, animals previously exposed to neonatal procedural pain (including repetitive handling and removal from litter) showed lower state anxiety but did not differ in trait anxiety, as compared with the undisturbed controls. These findings indicate that early-life procedural pain decreases state but not trait anxiety behavior in later life in a rodent model of repetitive needle pricking.

Impaired social cognition caused by perinatal protein malnutrition evokes neurodevelopmental disorder symptoms and is intergenerationally transmitted

Nutritional inadequacy before birth and during postnatal life can seriously interfere with brain development and lead to persistent deficits in learning and behavior. In this work, we asked if protein malnutrition affects domains of social cognition and if these phenotypes can be transmitted to the next generation. Female mice were fed with a normal or hypoproteic diet during pregnancy and lactation. After weaning, offspring were fed with a standard chow. Social interaction, social recognition memory, and dominance were evaluated in both sexes of F1 offspring and in the subsequent F2 generation. Glucose metabolism in the whole brain was analyzed through preclinical positron emission tomography. Genome-wide transcriptional analysis was performed in the medial prefrontal cortex followed by gene-ontology enrichment analysis. Compared with control animals, malnourished mice exhibited a deficit in social motivation and recognition memory and displayed a dominant phenotype. These altered behaviors, except for dominance, were transmitted to the next generation. Positron emission tomography analysis revealed lower glucose metabolism in the medial prefrontal cortex of F1 malnourished offspring. This brain region showed genome-wide transcriptional dysregulation, including 21 transcripts that overlapped with autism-associated genes. Our study cannot exclude that the lower maternal care provided by mothers exposed to a low-protein diet caused an additional impact on social cognition. Our results showed that maternal protein malnutrition dysregulates gene expression in the medial prefrontal cortex, promoting altered offspring behavior that was intergenerationally transmitted. These results support the hypothesis that early nutritional deficiency represents a risk factor for the emergence of symptoms associated with neurodevelopmental disorders.

Sex-specific behavioral and structural alterations caused by early-life stress in C57BL/6 and BTBR mice

Lately, the development of various mental illnesses, such as depression, personality disorders, and autism spectrum disorders, is often associated with traumatic events in childhood. Nonetheless, the mechanism giving rise to this predisposition is still unknown. Because the development of a disease often depends on a combination of a genetic background and environment, we decided to evaluate the effect of early-life stress on BTBR mice, which have behavioral, neuroanatomical, and physiological features of autism spectrum disorders. As early-life stress, we used prolonged separation of pups from their mothers in the first 2 weeks of life (3 h once a day). We assessed effects of the early-life stress on juvenile (postnatal day 23) and adolescent (postnatal days 37-38) male and female mice of strains C57BL/6 (B6) and BTBR. We found that in both strains, the early-life stress did not lead to changes in the level of social behavior, which is an important characteristic of autism-related behavior. Nonetheless, the early-life stress resulted in increased locomotor activity in juvenile BTBR mice. In adolescent mice, the stress early in life caused a low level of anxiety in B6 males and BTBR females and increased exploratory activity in adolescent BTBR males and females. In addition, adolescent B6 male and female mice with a history of the early-life stress tended to have a thinner motor cortex as assessed by magnetic resonance imaging. As compared to B6 mice, BTBR mice showed reduced levels of social behavior and exploratory activity but their level of locomotor activity was higher. BTBR mice had smaller whole-brain, cortical, and dorsal hippocampal volumes; decreased motor cortex thickness; and increased ventral-hippocampus volume as compared to B6 mice, and these parameters correlated with the level of exploratory behavior of BTBR mice. Overall, the effects of early postnatal stress are sex- and strain-dependent.

Limited contextual memory and transcriptional dysregulation in the medial prefrontal cortex of mice exposed to early protein malnutrition are intergenerationally transmitted

Memory contextualization is vital for the subsequent retrieval of relevant memories in specific situations and is a critical dimension of social cognition. The inability to properly contextualize information has been described as characteristic of psychiatric disorders like autism spectrum disorders, schizophrenia, and post-traumatic stress disorder. The exposure to early-life adversities, such as nutritional deficiency, increases the risk to trigger alterations in different domains of cognition related to those observed in mental diseases. In this work, we explored the consequences of exposure to perinatal protein malnutrition on contextual memory in a mouse model and assessed whether these consequences are transmitted to the next generation. Female mice were fed with a normal or hypoproteic diet during pregnancy and lactation. To evaluate contextual memory, the object-context mismatch test was performed in both sexes of F1 offspring and in the subsequent F2 generation. We observed that contextual memory was altered in mice of both sexes that had been subjected to maternal protein malnutrition and that the deficit in contextual memory was transmitted to the next generation. The basis of this alteration seems to be a transcriptional dysregulation of genes involved in the excitatory and inhibitory balance and immediate-early genes within the medial prefrontal cortex (mPFC) of both generations. The expression of genes encoding enzymes that regulate H3K27me3 levels was altered in the mPFC and partially in sperm of F1 malnourished mice. These results support the hypothesis that early nutritional deficiency represents a risk factor for the emergence of symptoms associated with mental disorders.

Maternal Separation Early in Life Alters the Expression of Genes Npas4 and Nr1d1 in Adult Female Mice: Correlation with Social Behavior

Early-life stress affects neuronal plasticity of the brain regions participating in the implementation of social behavior. Our previous studies have shown that brief and prolonged separation of pups from their mothers leads to enhanced social behavior in adult female mice. The goal of the present study was to characterize the expression of genes (which are engaged in synaptic plasticity) Egr1, Npas4, Arc, and Homer1 in the prefrontal cortex and dorsal hippocampus of adult female mice with a history of early-life stress. In addition, we evaluated the expression of stress-related genes: glucocorticoid and mineralocorticoid receptors (Nr3c1 and Nr3c2) and Nr1d1, which encodes a transcription factor (also known as REVERBα) modulating sociability and anxiety-related behavior. C57Bl/6 mice were exposed to either maternal separation (MS, 3 h once a day) or handling (HD, 15 min once a day) on postnatal days 2 through 14. In adulthood, the behavior of female mice was analyzed by some behavioral tests, and on the day after the testing of social behavior, we measured the gene expression. We found increased Npas4 expression only in the prefrontal cortex and higher Nr1d1 expression in both the prefrontal cortex and dorsal hippocampus of adult female mice with a history of MS. The expression of the studied genes did not change in HD female mice. The expression of stress-related genes Nr3c1 and Nr3c2 was unaltered in both groups. We propose that the upregulation of Npas4 and Nr1d1 in females with a history of early-life stress and the corresponding enhancement of social behavior may be regarded as an adaptation mechanism reversing possible aberrations caused by early-life stress.