Environmental enrichment attenuates chronic stress-induced disruptions in the gut microbiota, intestinal barrier, and brain.

Major depressive disorder is a prevalent mood disorder characterized by affective and cognitive impairment. Alterations in gut microbiota have been implicated in its pathophysiology, as stress-related microbial changes can disrupt intestinal barrier integrity and promote inflammatory activation along the microbiota-gut-brain axis. Although environmental enrichment (EE) has been shown to exert beneficial effects on stress-related behaviors, its influence on gut microbiota under chronic stress conditions remains unclear. Here, we examined the effects of EE on depressive-like behavior, stress-related molecular markers in the brain, gut microbiota composition, and intestinal barrier function in a chronic unpredictable mild stress (CUMS) mouse model. Following CUMS exposure for 5 weeks, mice were housed in either a standard environment or EE for 4 weeks. EE significantly reduced immobility time in the tail suspension test and restored tryptophan hydroxylase‑positive cell numbers in the dorsal raphe nucleus while reversing glucocorticoid receptor‑positive cell expression in the hippocampus. Microbiota analysis revealed that CUMS elevated alpha diversity and altered beta diversity, accompanied by increases in stress‑associated taxa-including Firmicutes, Desulfobacterota, Patescibacteria, and Ruminococcus-and reductions in beneficial taxa such as Akkermansia. EE prevented the CUMS-induced alterations, reestablishing microbial diversity and community structure toward control levels and partially reversing the taxonomic changes. In the colon, CUMS reduced claudin‑1 expression and increased IL‑6 and NF‑κB protein levels, reflecting impaired barrier integrity and enhanced inflammatory responses. EE attenuated the CUMS-induced intestinal disruptions, restoring tight junction protein expression and reducing inflammatory markers. In conclusion, EE may serve as a non‑pharmacological intervention capable of stabilizing the microbiota-gut-brain axis in depressive‑like states.