The behavioral phenotype of early life adversity: A 3-level meta-analysis of rodent studies

Altered cognitive performance is considered an intermediate phenotype mediating early life adversity (ELA) effects on later-life development of mental disorders, e.g. depression. Whereas most human studies are limited to correlational conclusions, rodent studies can prospectively investigate how ELA alters cognitive performance in several domains. Despite the volume of reports, there is no consensus on i) the behavioral domains being affected by ELA and ii) the extent of these effects. To test how ELA (here: aberrant maternal care) affects specific behavioral domains, we used a 3-level mixed-effect meta-analysis, and thoroughly explored heterogeneity with MetaForest, a novel machine-learning approach. Our results are based on >400 independent experiments, involving ∼8600 animals. Especially in males, ELA promotes memory formation during stressful learning but impairs non-stressful learning. Furthermore, ELA increases anxiety-like and decreases social behavior. The ELA phenotype was strongest when i) combined with other negative experiences ("hits"); ii) in rats; iii) in ELA models of ∼10days duration. All data is easily accessible with MaBapp (https://osf.io/ra947/), allowing researchers to run tailor-made meta-analyses, thereby revealing the optimal choice of experimental protocols and study power.

Stress and estrous cycle affect strategy but not performance of female C57BL/6J mice

Stress induces a switch in learning strategies of male C57BL/6J mice from predominantly spatial to more stimulus-response learning. To study generalization of these findings over sex, we investigated female C57BL/6J mice at three phases of the estrous cycle under non stress and acute (10 min) restraint stress conditions. On a circular hole board (CHB) task, about half of the naive female mice used spatial and stimulus-response strategies to solve the task. Under stress, female mice favored spatial over stimulus-response strategies, with 100% of female mice in the estrus phase. Performance expressed as latency to solve the task is only improved in stressed female mice in the estrus phase. We conclude that the use of learning strategies is influenced by sex and this difference between sexes is aggravated by acute stress.