Early-life maternal separation enhances vulnerability to adolescent social isolation: behavioral and structural...

Pathological aggression often results from the interaction between early-life adversity and later psychosocial stress. Maternal separation (MS) is an early-life stress model that increases vulnerability to aggression. However, social isolation (SI) is a paradigm for inducing aggressive behaviors and anterior cingulate cortex (ACC) alterations. Yet, the relationship between MS to adult behavior and ACC structure, and the modulatory role of SI during adolescence, remain unclear. This study examined the effects of MS, with or without SI, on aggression, anxiety, sensory sensitivity, and ACC morphology. Swiss albino pups underwent MS (4 h/day, PND2-20) and were later housed at PND46 for two weeks under standard or isolated conditions, which resulted four groups per sex: MS^-/SI^-, MS⁺/SI^-, MS^-/SI⁺, and MS⁺/SI⁺. Behavioral assessments were conducted in adulthood (PND60-62). The Resident-Intruder test indicated increased aggression exclusively in MS+/SI+ males, while social behavior declined in both sexes of MS⁺/SI^- and MS⁺/SI⁺. The elevated plus maze showed increased anxiety specifically in MS⁺/SI⁺ females. Mechanical allodynia, assessed by the von Frey test, was observed in MS+/SI- and MS+/SI+ groups, with no changes in thermal sensitivity measured by the hot plate test. Morphological analyses using Nissl and Golgi-Cox staining revealed ACC alterations in MS+/SI- and MS+/SI+ mice of both sexes, including reduced neuronal density, shorter dendritic length, diminished branching, lower spine density, and decreased arborization complexity of pyramidal neurons. These results highlight a synergistic effect between MS and SI: MS predisposes individuals to SI's detrimental impact, while SI amplifies MS induced behavioral and neurobiological alterations. The ACC emerges as a critical substrate mediating the cumulative effects of stress on pathological aggression and its comorbidities.