Abstract Chronic childhood stress is a prominent risk factor for developing mood disorders, yet mechanisms underlying this association remain unclear. Serotonin (5-HT) plays a crucial role in neurodevelopment and vulnerability to mood disorders. Maintenance of optimal 5-HT levels during early postnatal development is critical for the maturation of brain circuits. Developmental stress can alter the serotonin system, leading to chronic behavioral deficits. Yet, our understanding of the long-term impact of early life stress (ELS) on serotonin connectivity remains incomplete. Using a mouse model of chronic developmental stress, we sought to determine how ELS impacts brain-wide serotonin activity and behavior in adulthood. We established that adult female and male mice exposed to ELS during the first postnatal week show heightened anxiety-like behavior. Using in vivo fiber photometry and c-fos dependent activity mapping, we found that ELS enhances susceptibility to acute stress by disrupting the brain-wide functional connectivity of the raphe nucleus and the activity of dorsal raphe serotonin neuron population, in conjunction with a profound increase in the orbitofrontal cortex (OFC) activity. We further identified that 5-HT release in the medial OFC during environmental challenge is disrupted in mice exposed to ELS. Optogenetic stimulation of 5-HT terminals in the mOFC elicited an anxiolytic effect in ELS mice in a sex-dependent manner. Our findings hold significant insight into the mechanisms underlying long-term brain connectivity deficits induced by ELS, with potential implications for developing targeted stimulation-based treatments for affective disorders that arise from early life adversities.

Long-term impact of early life stress on serotonin connectivity