Vulnerability to stress-related disorders is considered to arise from disturbances during early neurodevelopment. Accumulating evidence highlights prefrontal cortex (PFC) circuits as key to the developmental origins of mood disorders. Among these, the projection from the PFC to the dorsal raphe nucleus (DRN)—the main source of forebrain serotonin (5-HT)—plays a central role in emotional regulation and stress responses. There is a mouse critical period for brain development, between postnatal days (P) 2 to 14, when environmental challenges can shape neural circuitry. Maternal separation (MS) is an early-life stress paradigm that induces emotional alterations in adulthood. We examined how MS affects the development and refinement of the PFC–DRN circuit during this critical period. We analyzed changes in PFC–DRN synaptic anatomy using high-resolution microscopy (Array Tomography), and assessed stress-induced activation of serotonergic DRN neurons. We also performed ex vivo patch-clamp recordings from 5-HT and GABAergic DRN neurons in MS mice across postnatal development. Our findings reveal that MS during the first two postnatal weeks disrupts the normal maturation of PFC inputs to the DRN and alters stress-dependent activation of 5-HT neurons. These results identify a key developmental window for PFC–DRN circuit formation, when early-life stress may induce maladaptive changes that could represent a neural substrate underlying increased vulnerability to psychiatric disorders.
