Mechanisms underlying the amplification of chronic restraint stress vulnerability in adult mice by adolescent social isolation stress and the ameliorative effect of light treatment.

Early-life stress is a critical determinant of vulnerability to later-life affective and cognitive dysfunction, yet the mechanisms through which adolescent adversity enhances adult stress susceptibility remain incompletely understood. Here, we employed a two-hit model combining adolescent social isolation stress (SIS) with adult chronic restraint stress (CRS) to examine how developmental stress interacts with adult stress exposure. SIS alone or CRS alone exerted minimal behavioral effects, whereas SIS followed by CRS markedly potentiated depression-like behaviors and impaired spatial and object recognition memory. Two-hit stress produced robust hippocampal neuroinflammatory responses, including increased astrocytic and microglial activation and elevated TNF-α, IL-1β, IL-6, and IL-17A levels. These inflammatory alterations were accompanied by pronounced suppression of the BDNF/TrkB/p-CREB signaling cascade, reduced synaptic protein expression, and diminished dendritic spine density and branching complexity in CA1 pyramidal neurons. Notably, light treatment (LT) administered during CRS exposure significantly reversed two-hit induced behavioral deficits, attenuated glial activation and cytokine upregulation, enhanced BDNF/TrkB and p-CREB signaling, and restored synaptic and structural plasticity. Together, these findings indicate that adolescent SIS primes the hippocampus for exaggerated neuroinflammatory and neuroplastic impairments following adult stress, thereby amplifying stress vulnerability. Furthermore, LT emerges as a safe non-pharmacological intervention capable of mitigating combined stress-induced emotional and cognitive dysfunction by targeting neuroinflammatory and neurotrophic pathways.