Differential effects of prenatal stress in female 5-HTT-deficient mice: towards molecular mechanisms of resilience

Highlights regarding prolactin in the dentate gyrus and hippocampus

Prolactin (PRL) is a pituitary hormone that has been typically related to lactogenesis in mammals. However, it has been described over 300 roles in the organism of vertebrae and its relationship with the central nervous system (CNS) is yet to be clarified. Mainly secreted by the pituitary gland, the source of prolactin in the CNS remains unclear, where some experiments suggest active transport via an unknown carrier or, on the contrary, PRL being synthesized on the brain. So far, it seems to be involved with neurogenesis, neuroprotection, maternal behavior and cognitive processes in the hippocampus and dentate gyrus, among other regions.

Pre-reproductive stress in adolescent female rats alters oocyte microRNA expression and offspring phenotypes: pharmacological interventions and putative mechanisms

Pre-reproductive stress (PRS) to adolescent female rats alters anxiogenic behavior in first (F1)- and second-generation (F2) offspring and increases mRNA expression of corticotropin-releasing factor receptor type 1 (Crhr1) in oocytes and in neonate offspring brain. Here, we ask whether the expression of Crhr1 and Crhr1-targeting microRNA is altered in brain, blood, and oocytes of exposed females and in the brain of their neonate and adult F1 and F2 offspring. In addition, we inquire whether maternal post-stress drug treatment reverses PRS-induced abnormalities in offspring. We find that PRS induces a selective increase in Crhr1-targeting mir-34a and mir-34c in blood and oocytes, while non-Crhr1 microRNA molecules remain unaltered. PRS induces similar microRNA changes in prefrontal cortex of F1 and F2 neonates. In adult animals, cortical Crhr1, but not mir-34, expression is affected by both maternal and direct stress exposure. Post-PRS fluoxetine (FLX) treatment increases pup mortality, and both FLX and the Crhr1 antagonist NBI 27914 reverse some of the effects of PRS and also have independent effects on F1 behavior and gene expression. PRS also alters behavior as well as gene and miRNA expression patterns in paternally derived F2 offspring, producing effects that are different from those previously found in maternally derived F2 offspring. These findings extend current knowledge on inter- and trans-generational transfer of stress effects, point to microRNA changes in stress-exposed oocytes as a potential mechanism, and highlight the consequences of post-stress pharmacological interventions in adolescence.

Identification of Cholecystokinin by Genome-Wide Profiling as Potential Mediator of Serotonin-Dependent Behavioral Effects of Maternal Separation in the Amygdala

Converging evidence suggests a role of serotonin (5-hydroxytryptamine, 5-HT) and tryptophan hydroxylase 2 (TPH2), the rate-limiting enzyme of 5-HT synthesis in the brain, in modulating long-term, neurobiological effects of early-life adversity. Here, we aimed at further elucidating the molecular mechanisms underlying this interaction, and its consequences for socio-emotional behaviors, with a focus on anxiety and social interaction. In this study, adult, male Tph2 null mutant (Tph2 -/-) and heterozygous (Tph2 +/-) mice, and their wildtype littermates (Tph2 +/+) were exposed to neonatal, maternal separation (MS) and screened for behavioral changes, followed by genome-wide RNA expression and DNA methylation profiling. In Tph2 -/- mice, brain 5-HT deficiency profoundly affected socio-emotional behaviors, i.e., decreased avoidance of the aversive open arms in the elevated plus-maze (EPM) as well as decreased prosocial and increased rule breaking behavior in the resident-intruder test when compared to their wildtype littermates. Tph2 +/- mice showed an ambiguous profile with context-dependent, behavioral responses. In the EPM they showed similar avoidance of the open arm but decreased prosocial and increased rule breaking behavior in the resident-intruder test when compared to their wildtype littermates. Notably, MS effects on behavior were subtle and depended on the Tph2 genotype, in particular increasing the observed avoidance of EPM open arms in wildtype and Tph2 +/- mice when compared to their Tph2 -/- littermates. On the genomic level, the interaction of Tph2 genotype with MS differentially affected the expression of numerous genes, of which a subset showed an overlap with DNA methylation profiles at corresponding loci. Remarkably, changes in methylation nearby and expression of the gene encoding cholecystokinin, which were inversely correlated to each other, were associated with variations in anxiety-related phenotypes. In conclusion, next to various behavioral alterations, we identified gene expression and DNA methylation profiles to be associated with TPH2 inactivation and its interaction with MS, suggesting a gene-by-environment interaction-dependent, modulatory function of brain 5-HT availability.

Impact of varying social experiences during life history on behaviour, gene expression, and vasopressin receptor gene methylation in mice

Both negative and positive social experiences during sensitive life phases profoundly shape brain and behaviour. Current research is therefore increasingly focusing on mechanisms mediating the interaction between varying life experiences and the epigenome. Here, male mice grew up under either adverse or beneficial conditions until adulthood, when they were subdivided into groups exposed to situations that either matched or mismatched previous conditions. It was investigated whether the resulting four life histories were associated with changes in anxiety-like behaviour, gene expression of selected genes involved in anxiety and stress circuits, and arginine vasopressin receptor 1a (Avpr1a) gene methylation. Varying experiences during life significantly modulated (1) anxiety-like behaviour; (2) hippocampal gene expression of Avpr1a, serotonin receptor 1a (Htr1a), monoamine oxidase A (Maoa), myelin basic protein (Mbp), glucocorticoid receptor (Nr3c1), growth hormone (Gh); and (3) hippocampal DNA methylation within the Avpr1a gene. Notably, mice experiencing early beneficial and later adverse conditions showed a most pronounced downregulation of Avpr1a expression, accompanied by low anxiety-like behaviour. This decrease in Avpr1a expression may have been, in part, a consequence of increased methylation in the Avpr1a gene. In summary, this study highlights the impact of interactive social experiences throughout life on the hippocampal epigenotype and associated behaviour.

Active coping of prenatally stressed rats in the forced swimming test: involvement of the Nurr1 gene

Depending on genetic predisposition, prenatal stress may result in vulnerability or resilience to develop psychiatric disorders in adulthood. Nurr1 is an immediate early gene, important in the brain for the stress response. We tested the hypothesis that prenatal stress and the decrease of hippocampal Nurr1 alter offspring behavioral responses in the forced swimming test (FST). Pregnant Wistar rats were exposed to restraint stress (45 min, thrice daily) from gestation day 14. Prenatally stressed (PS) and non-prenatally stressed (NPS) male offspring were treated bilaterally with a Nurr1 antisense oligodeoxynucleotide (ODN; or control) into the hippocampus at 97 d of age. After 1 h, the rats were exposed to the FST (acute stressor) to analyze their behavioral responses. Thirty minutes after the FST, we analyzed the gene expression of Nurr1, Bdnf and Nr3c1 (genes for Nurr1, brain-derived neurotrophic factor (BDNF) and glucocorticoid receptor (GR), respectively) in the hippocampus, prefrontal cortex (PFC) and hypothalamus. Results showed that the decrease of hippocampal Nurr1 after the antisense ODN in adult NPS rats induces immobility (indicating depressive-like behavior). The PS adult rats, including the group with decreased hippocampal Nurr1, presented low immobility in the FST. This low immobility was concordant with maintenance of Nurr1 and Bdnf expression levels in the three analyzed brain regions; Nr3c1 gene expression was also maintained in the PFC and hypothalamus. These findings suggest that Nurr1 and associated genes could participate in the brain modifications induced by prenatal stress, allowing active coping (resilience) with acute stress in adulthood.

Actions of Prolactin in the Brain: From Physiological Adaptations to Stress and Neurogenesis to Psychopathology

Prolactin (PRL) is one of the most versatile hormones known. It is considered an adaptive hormone due to the key roles it plays in the modulation of the stress response and during pregnancy and lactation. Within the brain, PRL acts as a neuropeptide to promote physiological responses related to reproduction, stress adaptation, neurogenesis, and neuroprotection. The action of PRL on the nervous system contributes to the wide array of changes that occur in the female brain during pregnancy and result in the attenuation of the hypothalamic-pituitary-adrenal axis. Together, all these changes promote behavioral and physiological adaptations of the new mother to enable reproductive success. Brain adaptations driven by PRL are also important for the regulation of maternal emotionality and well-being. PRL also affects the male brain during the stress response, but its effects have been less studied. PRL regulates neurogenesis both in the subventricular zone and in the hippocampus. Therefore, alterations in the PRL system due to stress or exposure to substances that reduce neurogenesis or other conditions, could contribute to maladaptive responses and pathological behavioral outcomes. Here, we review the PRL system and the role it plays in the modulation of stress response and emotion regulation. We discuss the effects of PRL on neurogenesis and neuroprotection, the putative neuronal mechanisms underlying these effects, and their contribution to the onset of psychopathological states such as depression.