An early experience of mild adversity involving temporary denial of maternal contact affects the serotonergic system of adult male rats and leads to a depressive-like phenotype and inability to adapt to a chronic social stress.

Adverse early life experiences can affect adaptability to chronic stressors and lead to depressive-like behaviors in animal models. We employed an early experience model in which rat pups during postnatal days 10–13 are exposed to a T-maze in which they learn the location of their mother motivated by the rewarding stimulus of maternal contact; one group of rats receives the expected reward, by being allowed contact with the mother upon finding her, while the other group is temporarily denied this contact (Denied Expected Reward, DER), thus experiencing mild adversity. The results presented herein show that the DER early life experience results in a depressive-like phenotype in adulthood, as indicated by the absence of sucrose preference –anhedonia- exhibited by these animals, in adulthood. Following exposure to a chronic social stress (CSS), DER male rats were unable to adapt, evident by reduced general locomotion and time spent in the centre of an open field which indicate anxiety and/or decreased motivation for exploration. They also exhibited increased immobility time in the forced swimming test, suggesting a passive coping strategy. The depressive-like and anxious phenotype of the DER males was accompanied by changes in the serotonergic system, such as lower serotonin levels, higher serotonin turnover and higher levels of the type 1 serotonin receptor in the hippocampus. Our results corroborate findings showing that early life adversity disturbs behavioral regulation in adulthood. They also suggest that even mild adversity, if it involves intervention in mother-offspring interactions, can be sufficient to compromise adaptability.